In my experience as a Mac + iOS user and developer for the past 8 years, and for the rest of my life before that on Microsoft platforms, Apple's record has been much better compared to Microsoft re: abandoning APIs or pulling the rug out from under devs.
Take NSDocument for example. It has been around for almost 20 years. All you usually needed to do was inherit from it and adopt its interface and your app would often get new features for free with each release of macOS, like autosaving and version histories.
The foundations for multiple UI appearances have been there since macOS 10.10 Yosemite, 4 years ago. They were hard not to notice.
As a user and developer I'm glad they took the time to work out a sensible implementation that is very easy to adopt now and makes good sense for the future. It definitely seems to be in a better spot than Windows' support for multiple appearances currently, if it has any.
If you see the WWDC sessions on Dark Mode, you might come to appreciate the thought and effort they've put into it:
[0] Introducing Dark Mode: https://developer.apple.com/videos/play/wwdc2018/210/
[1] Advanced Dark Mode: https://developer.apple.com/videos/play/wwdc2018/218/