Comment by p_l

The true success of WinAPI, including originally with Windows, is that it provided stable ABI from version to version, and didn't lock you into any language.

The Unix world was lazy about it because of the approach of recompiling across somewhat source compatible systems thanks to POSIX, so there was reasonably fast portability if you didn't go too far off the beaten path.

But doing anything other than C (w/ Cfront maybe) and Fortran and Pascal was a problem, even without binary compat. Even from version to version (legacy of which we now have in glibc breaking binary compat all the time).

Microsoft went hard on the idea that if you bought/build a program for Windows version X, it would run on version X+1. You didn't have to buy a special upgraded version. You could update easier.

The same approach later drove introduction of things like PC System Design Guide and ACPI so you could just upgrade your computers instead of waiting for special OS upgrade just to boot (like it was common in other platforms, including Mac, VMS, and Unix workstation world).

Design wise, GUI parts of WinAPI aren't all that different from working with X intrinsics etc libraries (i.e. the parts above raw xlib)