You want to use Gnome? Use Gnome. You want to use KDE? Use KDE. I3? ditto. Etc.
Oh, some random GTK app will look weird on KDE? Tough. Every Windows app looks different, and even MS's apps always felt out of place on their own OS. And with the proliferation of Electron apps, nothing ever looks "native" anymore.
> while I don't know if anything that has been built 2 Ubuntu LTS releases ago still works
Maybe, maybe not. But that's also an issue on MacOS. And while Windows prides itself on backwards compatibility (and mostly delivers), that doesn't come free.
There's also hardware backwards compatibility. Many devices that used to work with older versions of Windows or MacOS just don't work anymore. Even those that never required custom drivers from their manufacturer. On Linux, if it works once, it'll usually keep on working. I have such an issue: a PCIe USB 3 card, so it isn't even something that old. Used to work with MS Win 10 provided drivers, now (win 11) it's unusable.