a mobile OS that basically eschews backwards compatibility

I have an app built for Android 4 running on my Android 15 device. It looks ugly but it works. Of course other apps will not be so lucky, but some backwards compat is absolutely there.

a desktop OS that can still run 30 year old applications

Not really, Microsoft is steadily breaking old stuff. For example lot of 10-15 year old software that was doing something hardware-related would be broken now due to driver signing changes/restrictions (e.g. WinRing0 things).