> Agree, but it isn't like OSes also don't change their API space or behaviors.

They change, but more slowly with better backward compatibility, but more important is when they change. In a corporate environment they might update the OS once a decade and on a schedule you control which is much more manageable than relying on a layer that updates itself every 6 weeks.

> Unless one only cares about fossilized UNIX and Win32.

You say that like stability is a bad thing.

Stability is good, but not when it becomes an obstacle for advancing the state of art in OS development, as many cling to the ways of yore and we get CLI apps with coloured terminals as improvement or WinForms database frontends in bigger screens.