It's really hard to put any single root cause on it. Firstly, mobile OSes are massive beasts with lots of different teams working on it. Co-ordinating this becomes very difficult. Additionally, you typically get about 1 build a day which means there's a long lead time to find a bug that's managed to propagate out and then a long lead time for it to get fixed (at least 1 day). I think this decrease in quality has been seen across all OS vendors, it's just that Apple has become one of the dominant OSes so there's more press about it than there was about previous Apple releases.
So now you have a larger volume of bugs since bug count is based on the amount of code written, assuming best case that developer quality is roughly consistent across all teams. Since you have a larger volume and only so many hours in a day, trying to ship all software at once at an annual cadence means your bug count will go up regardless of how much QA you throw at the problem (since in addition to finding issues you have to fix them).
Automated testing might help because you could catch bugs before they're even submitted. Well, automated testing is still very hard on these devices when you're building the OS. For example, how do you do something as simple as build & run unit tests? If the answer involves loading code onto a mobile device that means (at Apple scale) you need a farm of devices. Moreover, your test isn't valuable if it's running against yesterday's OS. Might not even build/run because you've now acquired a dependency on an API made available in the new OS. So now you have to co-ordinate building the software using an SDK that may be unstable (build breakages happen), run it on an OS you somehow have to pre-validate works otherwise & then have some confidence that the bug found was introduced by your commit & not some combination of build issue, compiler bug (oh yeah - the compiler changes drastically too while your building your next OS), or downstream dependency bug. Finally, on top of all this you need to make sure you can recover bricked devices in an automated fashion so that the maintenance overhead of your automated testing cluster is manageable. All of these aren't impossible problems to solve. The problem is that it requires a lot of effort & physical infrastructure. Additionally, Apple has had a very ad-hoc development environment where no 2 teams are the same (it was slowly changing to unify large pieces of infrastructure when I left) which complicates the ability to roll out central infrastructure. Additionally, there are different needs - the kernel has different testing requirements/strategies from system daemons, from drivers from 1p apps from backward compatibility testing.
When I left a couple of years ago Craig had actually directed his teams to improve the automated testing infrastructure but at the end of the day the software is growing more quickly than the investment that was made in QA. There's also only so quickly you can roll out central automation infrastructure to an organization as large as Apple so it's not as simple as "do more QA" or "add more QA resources" not to mention that "QA resources" in automated testing typically requires very skilled software engineers to design & build the systems. Allowing a longer bake time is exactly the right move here as it recognizes the need to decrease the volume of bugs while infrastructure catches up.
However, the contradiction is that single giant releases help in three ways. The first is it enables you to make 1 big splash of a years worth of work to customers rather than it being dribbled out of the course of a year across different components, which is super important because it dove-tails into the release of the next flagship phone. It also makes the messaging because you can build themes for your releases that are co-ordinated across many apps/services.
The second is that it co-ordinates large-scale changes across the company (e.g. major UI redesign, some major UX improvement, etc) which would be more difficult to co-ordinate with smaller releases or if apps were split out.
The third is that leaks/previews can be better controlled as managing a bunch of different releases that contain functional differences is harder than just maintaining bug fixes (i.e. prepping UI changes for 50 different apps that are released throughout the year in preparation for the next OS that makes it possible for those apps to run).
Finally, it simplifies development in a sense because you don't have to worry about 1P backwards compatibility. 1P apps don't have to worry about supporting more than 1 OS and OS changes can confidently break 1P apps (within reason) with new APIs without worrying about apps that haven't been updated. The latter part about breaking apps doesn't matter as much if the apps are bundled with the OS unless those apps can also deliver updates via the App Store as now you have to launch vehicles that can be tricky to co-ordinate.
Yes, a lot of this is pure business reasons for why the SW is co-ordinated but that doesn't mean that it's not valid. Business practices & SW are symbiotic aspects of a tech company as neither can exist without the other. Everything is a list of tradeoffs and priorities. Apple is apparently first experimenting to see if they can keep all the business pros of their current approach by altering their program management practices which they've already done - iOS & OSX today have far more dot releases in 3 months than they used to between major versions. Reducing scope is the approach they're taking in the interim to make the major releases more stable - will be interesting to see if this is a long-term shift or just an interim one until their QA infrastructure investments catch up to be able to keep up with development. If those approaches fail & the brand becomes at risk that may outweigh the other business reasons & result in independent app & OS releases (seems unlikely IMO).
Hmm... I think Kim came in towards the time I was leaving. Last name? When I was there our immediate QA was embedded within SW & reported up through the SW chain. Craig took over all OS SW development early 2015 IIRC. Have things changed? FWIW software quality of iOS has always been a concern in the media and I don't see that iOS 11 is particularly better or worse than other releases. iOS 8 had horrible performance issues for example to the point where iPhone 4S lifetime had to be artificially extended to iOS 9 instead of being EOL'ed in iOS 8 as originally planned. Every release has had its own share of embarrassing/severe bugs. Except for maybe iOS 1.0 which had no 3P features, a very small feature-set, & no competition to be evaluated against, each iPhone release has always been received as having stability issues, at least initially.
> The iOS 5 update was the subject of criticism for iPhone 4S users, as the upgrade caused problems with battery life, failures of SIM cards, and echoes during phone calls.
https://en.wikipedia.org/wiki/IOS_5
> A study by Apteligent (formerly Crittercism) found that the rate at which apps crashed in their tests was 3.56% on iOS 8, higher than the 2% found on iOS 7.1.[81]
> Forbes published several articles focusing on problems in iOS 8 regarding Wi-Fi and battery,[82] Bluetooth,[83] and calendar.[84]
https://en.wikipedia.org/wiki/IOS_8
Except for iOS6 & 7 which appear to have had a milder reaction, if anything I think iOS 11 may be a more stable release compared to iOS 8, 9, & 10 in that while there are more frustrating UI bugs you can hit (auto-correct, iMessage message ordering) but the overall performance & stability hasn't suffered as badly. Also iOS has knock-on impressions from OSX stability which has had similar UI embarrassments + a couple of security-related missteps.
My point is I don't think there's any single particular cause that's identifiable as the reason behind poor SW quality. If it were the hundreds if not thousands of really smart people Apple employs would have solved the problem. At this point it's all higher-level issues & more often than not tradeoffs (e.g. reducing the features you ship every year & going away from a waterfall development model)
So now you have a larger volume of bugs since bug count is based on the amount of code written, assuming best case that developer quality is roughly consistent across all teams. Since you have a larger volume and only so many hours in a day, trying to ship all software at once at an annual cadence means your bug count will go up regardless of how much QA you throw at the problem (since in addition to finding issues you have to fix them).
Automated testing might help because you could catch bugs before they're even submitted. Well, automated testing is still very hard on these devices when you're building the OS. For example, how do you do something as simple as build & run unit tests? If the answer involves loading code onto a mobile device that means (at Apple scale) you need a farm of devices. Moreover, your test isn't valuable if it's running against yesterday's OS. Might not even build/run because you've now acquired a dependency on an API made available in the new OS. So now you have to co-ordinate building the software using an SDK that may be unstable (build breakages happen), run it on an OS you somehow have to pre-validate works otherwise & then have some confidence that the bug found was introduced by your commit & not some combination of build issue, compiler bug (oh yeah - the compiler changes drastically too while your building your next OS), or downstream dependency bug. Finally, on top of all this you need to make sure you can recover bricked devices in an automated fashion so that the maintenance overhead of your automated testing cluster is manageable. All of these aren't impossible problems to solve. The problem is that it requires a lot of effort & physical infrastructure. Additionally, Apple has had a very ad-hoc development environment where no 2 teams are the same (it was slowly changing to unify large pieces of infrastructure when I left) which complicates the ability to roll out central infrastructure. Additionally, there are different needs - the kernel has different testing requirements/strategies from system daemons, from drivers from 1p apps from backward compatibility testing.
When I left a couple of years ago Craig had actually directed his teams to improve the automated testing infrastructure but at the end of the day the software is growing more quickly than the investment that was made in QA. There's also only so quickly you can roll out central automation infrastructure to an organization as large as Apple so it's not as simple as "do more QA" or "add more QA resources" not to mention that "QA resources" in automated testing typically requires very skilled software engineers to design & build the systems. Allowing a longer bake time is exactly the right move here as it recognizes the need to decrease the volume of bugs while infrastructure catches up.