What is good for customers and developers might not be good for Apple, and vice versa.
(A separate issue are the user docs, e.g. how to use Finder. I guess they are good enough, I haven't found the need for such docs.)
Obviously, Apple isn't hurting, but it can take a long time for anyone to notice that a huge tree is rotten in the middle. I don't know if that's true of Apple; they've been pretty successful in the transition to mobile, and their iOS ecosystem seems very healthy, but, if the terrible docs are a company-wide issue, it might be a slow-moving disaster. Docs are unsexy enough to where I could believe Apple would completely fuck them up, just because nobody wants to work on the unsexy stuff and nobody thinks about it when things are going so well for the company.
For example, macOS is themeable under the hood. Nearly all Cocoa controls are defined as vector graphics in an "art file", rather than hardcoded in the library. This was probably not done with themability in mind, but is just a side effect of good engineering. I bet there was a meeting where the engineers said "we have this great feature where we can change the theme of all Cocoa apps", and it was decided to not expose that feature for reasons like "brand recognition" and having all macs look alike for support reasons. I'm pretty confident that happened because Microsoft did the same and locked down themes with Windows XP (and somebody stated their reasoning), and also the GNOME project removed theming from the main UI and crammed it into a scary "tweak" tool (one dev said basically that theming is not a desired feature for them).
Similar logic applies to other things, like the kernel interface.
I fully understand why they come to such decisions - but I as a user and developer (and interested in "hacky" things) would love these things to be documented.
I don't think the status it is hurting their bottom-line, or the quality of mac apps (yet). Cocoa stuff seems to be documented decently enough. But I agree, they have to be careful if they don't want to fall behind.
And AmigaOS was a shining beacon of efficiently surfacing apps online early on. Aminet  provided a robust mirror system and ability to browse a big catalog (still online and updated) of downloadable AmigaOS software. It was a large part in letting AmigaOS remain viable for users much longer than it otherwise would have for most.
In fact, part of the point I didn't put across very clearly is that things like Aminet demonstrates how important that ecosystem is. Amiga would have become useless to most users far earlier if we didn't have incredibly well curated sets of applications, and amazing dedication to maintenance (e.g. even today people are releasing their own bug-fixed versions of system libraries and the like) - it took a lot of effort to try to compensate, and it still wasn't enough.
If they want users... then you kind of need to have even low level documentation. Because the kind of serious apps and third party feature adds that make an OS bearable to use depend on them.
If you have 50% of your OS programmers working on an OS that only 10% of your users use, seems like a waste
Indeed, if they have different APIs on the 2 platforms for the same functionality, you can probably bet that pretty soon one will be ported across to the other and the other obsoleted. Recent examples: PDFs, Bluetooth, Media SDKs (AV Foundation etc).