Microsoft breaks ABIs and userland apps all the fucking time. Why do you think so many people complain about Vista? It doesn't run a lot of apps that worked perfectly in XP (even if you install/run them in administrator mode or in XP compatibility mode). Hell, do any of you actually remember when XP came people? People bitched about it just as much because it broke a ton of apps that worked just fine in 95/98. Windows 7 can't run some apps that Vista handled just fine.
Windows maintains stable ABIs *where it can* because Real Users kind of prefer not having half their desktop stack break every 6 months while proclaiming the endless breakage as "true innovation!"