I think it's worse than that: we're now suffering being a "supported but deprioritised platform" for a cross platform GUI.
AppKit was developed for the Mac from the ground up. All effort that went into it was to make the Mac as good as possible. Experience from that went into making UIKit, which was made to be as good as possible for iPhone. Focus on iPhone made the Mac suffer somewhat from a lack of resources, but AppKit was still a rock solid foundation.
Swift UI is primarily made for iPhone. It's secondarily made for iPad. I'm willing to bet that almost all the effort that goes into it is focused on making iPhone and iPad apps better. And it is succeeding there, to some degree (though not without its own issues; especially now with iOS 26). Mac support, however, is clearly an afterthought. Yet it's now the foundation of everything in macOS.
It's not too dissimilar from what it would look like if Apple had decided to rewrite large swathes of the system in GTK when the GTK developers only really care about how well GTK works in a GNOME desktop.
I still don't understand who actually use iOS/iPadOS apps heavily.
Everyone I know just try to do stuff with them and at some point go find a computer to actually get shit done, frustrated with how backward everything is on those platforms.
Now they want to make the Mac the same. What the fuck.