[Wowfunhappy]

Wouldn't the increasing dominance of mobile platforms and prevalence of macOS be a counterexample to theory?

It depends on how you define "replace", I suppose, but Windows really seems to have less and less importance in a lot of spaces.

[spijdar]

Neither mobile platforms nor MacOS are really trying to "replace" Windows, never have been. They have some overlapping markets, but the target has really been very different all around.

On the other hand, Apple has been very careful to include this sort of "backwards compatibility" in all their systems for a while. It's usually not maintained as long as Microsoft does, but from m68k -> ppc -> i386 -> amd64 -> armv8 they're always supported at least the last architecture and runtime, so early OS X could run apps from System 7/8/9.