X11 isn't backwards compatible with DX9, only 10. They are two entirely separate entities so you can't not support DX9, when the massive majority of hardware is still dx9, DX9 will have to be supported in games for a couple years at least, after that, some game makers will(hopefully) say they have to stop dx9 at some point and as their new game " game x" is so good graphically, theres no point going DX9 aswell as theres no dx9 card powerful enough to run it on lowest settings.
Once the first company gives up on DX11, which will pee off a lot of people, every other company will jump on the bandwagon as making a game for both DX versions will be a pain in the ass.