Honestly, I think the XBox 360 will have difficulty getting to 20 Million in North America and 35 Million worldwide before Microsoft is put into a position where they have to release another console.
If you look at the line-up of the XBox 360 (past, present and future) most of its high quality third party games are also available on the PC; to a certain extent you can say that the XBox 360 is leeching a lot of support from the PC. In 2009, the XBox 360 will be similar in performance to the minimum requirements of most PC games and in 2010 it will be below the system requirements of most PC games. What this means is that publishers would have to look at today's sales numbers and think it was worthwhile to produce a (potentially expensive) XBox 360 version of their games which seems unlikely due to its sales.
Edit: I'm not saying the XBox 360 will see no support, just that they will lose a lot of the support they currently have.
