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).
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.
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).