We're going in circles now. I can now again tell you that desktops' backwards compat is fine and XMMS still works. I can even point you to all the cases where Linux breaks compat all the time (like V4L, OSS, various /dev and /proc schemes) and then you can go back and point me at mailing list threads about people claiming it's important. Then I can point you at http://httpd.apache.org/docs/2.4/developer/new_api_2_4.html or http://www.uberphones.com/2010/11/angry-birds-android-com... or Python3 and you can find even more examples of where you think backwards compatibility rocks when it's as bad or worse as the "desktop".
Or we can go and play the fun vs money game some more. Where you can point at successful Windows Open Source apps and claim they do it right (you should pick better examples than AviSynth though) and I will point out where they completely fail (but my grandma fails at writing AviSynth scripts, she can use Premiere just fine!)
But you won't convince me that "the desktop" is at fault here. It is and will forever be a distro-level problem. Distros provide the ABI and API to the developers, not Gnome or KDE. Both Gnome and KDE keep this compatibility just fine. And they could easily do better if the distros using them deemed it the tiniest bit important. But they don't.