Because requirements change and you can't keep API's sane when only bolting on new functionality. You can disagree about the support timeframe, but in 10-15 years time we've gotten used to high resolution screens, HDR, HFR, multitouch etc. If you don't ever change API's you're leaving a lot of potential on the table.

Windows added all those functions without breaking compatibility. Yes, applications might have to be modified to take advantage of new features that make sense for the application in question, but that doesn’t imply that they should have to switch toolkits.

While Windows doesn’t provide a unified GUI toolkit, win32 UI controls of different generations can be mixed freely within an application, enabling gradual upgrades.

Windows is a paid operating system with thousands of developers working on it. GTK has 3 regular developers. Please step up if you're going to complain.

I guess that’s why the year of the Linux desktop will never come.