This might be a contributing issue, but it seems this is far from the only reason why Android has such shit upgrade lifetimes.
Android itself doesn't have a strict requirement for an updated kernel. It will typically run fine on the original kernel tree the SoC launched with.
OnePlus occasionally does this, they are currently rolling out Android 10 for the OnePlus 5 and 5T, based on the original kernel tree. That's a 3 year old device (and I hate that I'm using a 3 year old device as my best example)
It seems like most Android vendors simply choose not to offer extended updates.
Android device manufacturers make their money when you buy a new device. Apple, on the other hand, makes money when you buy apps (in addition to making money on device sales I'm sure). They have an incentive to make sure that the latest OS runs on as many devices as possible, so that developers can release their apps to a wider audience, and so that Apple can make more money. Google wants apps to run on as many devices as possible, so that's why they bend over backwards to support older devices with their SDK. Samsung doesn't have any incentive to update the Galaxy S6 since that won't bring them any additional revenue.
Wasn't Project Treble created a few years back to solve this problem? I'm wondering what the current state of it is, because I haven't seen the effects of it yet.
The postmarketOS project is trying to port these old Android devices to run on a mainline kernel, wherever feasible. It's non-trivial work, especially if you expect to submit patches upstream - I can see OEM SoC manufacturers running into issues with that. The whole Device Tree arrangement was intended to simplify this work, but a lot of hardware still comes with plenty of weird, one-off, entirely undocumented hacks. Sometimes it's a wonder how these devices can even boot at all!
Kinda explains why Nvidia was probably the best when it came to Android updates from my experience. They made the SOC themselves and had access to the drivers
I have no ability to contribute to this technical analysis, other than to say Apple would never expect an average user to know what a "kernel" is or why it has anything to do with their phone being out of date. I'm sure 100% valid issue here but this is like telling a customer about your back office problems while ignoring their needs. Phones and computers are nice when they're as simple to use as a toaster.
Google doesn't expect an average user to know that either. The difference from such a user's perspectice is just that Android devices become unable to run the most recent software more quickly than iOS ones. The post above is discussing the underlying causes of that. You are on a website called "Hacker News".
So the next version of android comes along and requires a newer kernel, and the phones built around these chipsets can't be updated.
The chipset folks see no new revenue for forward-porting their hacked up kernels and drivers, and therefore aren't interested in updating.
That is the problem the Android ecosystem needs to solve.
Because Apple controls the full stack, they have no problem forward porting any custom changes that they need. It's all the same codebase.