> Is it Apple’s responsibility to maintain X on the Mac, though?
Yes and no.
So POSIX and the Unix Specification(TM) are standards thanks to the US federal government putting their foot-down and mandating that major contractors (like IBM, Sun, HP, etc) stop being incompatible with each other - and in-practice this also means having to support X.
When OS X launched Apple loved to tell everyone that it was a true Unix(TM) system - (after-all, they did spend the money to get it certified as a real Unix system), and this meant that OS X-based systems were suddenly candidates for major US and international scientific and research contracts and gigs - which dovtailed nicely with the hype around their boasts of the PowerMac G5's literal supercomputer performance (and indeed: literal supercomputers comprised of racks of G5 machines in a bunch of US Navy Labs).
Now, in the 2020s, it's clear that since the iPhone dominated their revenue that Apple is no-longer really interested in being part of government supercomputing and scientific computing contracts - the positive PR and technical credibility they gain from all of those government projects is insignificant compared to the cash revenue value of simply running another iPhone ad-campaign. The 2013 Mac Pro was solid proof that Apple does not value the high-end desktop and workstation computing markets anymore. Apple seems content to even let Windows take away their previously impentrable hold on the desktop video-editing market: whoever was using FCP on a MP in 2006 is almost certainly using Adobe Premier on Windows today.
...having said all of that, I was surprised to see that macOS 12.0 Monterey is still an official Unix(TM) certified product: https://www.opengroup.org/openbrand/register/brand3673.htm - despite Apple's deprecation (and obsolescence?) of their support for X.
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So my point, if anything, is that if Apple wants to maintain its credibility as a high-end computing systems provider, it needs to revert back to doing far more than paying lip-service to being a Unix vendor - and that includes bringing native support for X (and some form of compatibility with the rest of the Unix/Linux GUI ecosystem) back into macOS - but if Apple continues to remain uninterested in selling themselves as a professional workstation vendor (which I feel is implicit in how they've been locking-down macOS and their Apple Silicon-based machines) then you're right, they don't have any responsibility to maintain compatibility with X.
As it is, I don't know what Apple wants - or even what Apple would do in the event they lose their cash-cow and grip on the world when the iPhone is somehow obsoleted by a competitor. Of course if that happens then they'll quickly reverse-course and try to make-up for all of their substandard devX moments.
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> That said, I’ve never regretted choosing it as my preferred platform, despite thirty years of insults and put-downs from other tech folks.
Pardon my asking, but I'm especially curious - what kind of software do you write? Unless you work for major creative-tools companies like Adobe - or a boutique Mac shop like Panic nor indie Mac App Store dev - I have trouble imagining what market there is for other types of software on the Mac. Whatever native macOS / Cocoa line-of-business software there was over the past couple of decades will have certainly transitions over to being a SaaS web-app by now.