The economics make the reasoning obvious, though.

With closed source IP, every bit of support, from bug fixes, to feature requests, to compatibility fixes to integrate with newer mainline/foundational tooling, costs money.

With open source projects (and in particular ones like Linux where there's a huge number of contributors and interested parties), support for would-be niche facilities can keep going as long as there's someone with the knowledge and spare time to do it.

AFAIK, Linux has a policy that any change you make must not break existing kernel features, and if it does, you have to fix them yourself.

With that said, kernel maintainers have recently indicated that some unused subsystems are likely to be removed soon, as AI is now finding (real) security vulnerabilities in them that nobody is willing to fix.

There's somewhere in the ballpark of 166,000 employees at Apple, just unfathomable scale [1]. It is not unreasonable to ask that someone specific is responsible for each particular small feature and ensuring it keeps working. Trying to apply an economic analysis to such a "free as in beer" operating system does not seem to work well. Consider the question of "how many small holes can you have in your wooden sailing ship"?

[1] https://stockanalysis.com/stocks/aapl/employees/

Ideally, at a certain point, you'd have some sort of upstream FLOSS project where you could let John Q. Public do that sort of low-level, maintenance-only stuff, while the proprietary "value adds" are closed source, until it becomes financially attractive to FLOSS them.

IIRC, that could exist for MacOS in the form of Darwin.

The economics make the reasoning obvious, though

These arguments fall apart when you remember that Apple has several trillion dollars at hand. It's not some shoestring startup.