It's not just apple, its everything in big tech. Venmo just released an update where they moved key workflows around for no reason at all, and now I don't have muscle memory for the app anymore, but I'm sure some project manager justified it with telemetry and got a huge bonus for rolling out an update and showing downloads grew by 1% (which they probably would have anyway).
But more importantly, they existence of change is inherently important to the large-scale advancement of computing over time. Even if it is the case that a certain UI overhaul of a major operating system harmed productivity, the solution is not “permanently halt all software changes after this specific version that I have learned and enjoy using.” We ought to reject arguments that forced stasis is the solution any time changes introduce risk.
We also ought to reject the idea that forced change is the solution.
If you'd let people mix and match elements from different eras of macOS, I'm sure you'd see some patterns regardless of when people got into Macs; similar to how people have lots of abstract opinions about architecture, but somehow the tourist buses always stop at the same cozy-looking old towns. Beauty and usability are not entirely subjective.
Look at the MacOS release feature lists before and after the iPhone. It's pretty clear that the Mac teams got cannibalized and didn't recover for years.
Mobile platforms benefit more from high contrast, very simple and flat designs.
Beyond just the touch screen convenience features -- A significant portion of users is seeig their UI under conditions of extreme sunlight, water droplets, or cracked screen at any given time. These things all inform design choices for mobile.
Now then, why is Apple making the same mistake as Microsoft Windows 8 by forcing these design elements onto Desktop and Laptop market segments? Apple doesn't really think of the PC market much. According to their financial reports, they make more money selling chargers for their mobile devices than they do on the entire PC market.
The answer, I assume, is apathy.
This design move is at this point getting to be a decade old. So this explanation from you makes zero sense.
I was mostly describing my feelings and guesses of it.