To answer the question, if Kinect becomes a succes PS3 will become the only remaining core console. :P
No seriously though, i don't think any Xbox 360 gamer should want it to become a massive succes, it should just do ''ok''. Because if it does become massive, X360 will become very casual orientated.
People usually make this assumption because of what happened with the wii. But that's wrong.
Wii game library problems are mainly due to its low hardware specs. Most core games today are multiplatform, 3rd party exclusives just aren't viable anymore, so outside of those 2-3/year AAA 1st pary games it's all multiplatform. If the wii was able to catch those multiplatform games it would replicate the library of the playstation2. PS2 had is fair share of shovelware and I never heard anyone complaing about the casualness. But the wii can't do that, because you usually can't port ps360pc games to the wii, at least not without butchering them.