Sure, I know all that, but it’s ironic to me that Unix, which was boasted as the epitome of portability once with C, POSIX, X11, X/Open and whatnot, actually struggles with backwards compatibility while Microsoft, the notorious developer of a closed-source, locked-in platform, has managed to become the key to portability :)

That's source-level portability, while this is binary portability. Even early on, MS put in lots of effort to ensure that old applications kept working.