Can somebody help me understand this no backwards compatibility nonsense

I understand that both Microsoft and Sony went with a new architecture which can't allow for direct backwards compatibility, but what I don't understand is this "new architecture" is x86 which most games have already been cross platform developed for (hello PC games).

For the few console exclusive games, I could see it requiring a port, but shouldn't this theoretically mean that the entire catalog of PC games should easily be playable on the new console making xbox 360 backwards compatibility basically a non-issue? Am I missing something???