I don't really see how anyone can think that the 360 will hit 50 million. It's approaching its forth year, and is only just going to be past 20 million (about 23 million if my calculations are correct). NOTE: This year is determined by launch date, not calendar years, so that's sometime in late November. Compare that to the Xbox and GameCube at a similar point in their lifes of 14.8 million (Xbox) and 14.2 million (GameCube), and then compare that to what their ending lifetime totals were: 24 million (Xbox), and 21 million (GameCube). The Xbox was cut short because of the problems of manufacturing it, but the GameCube wasn't cut short, and it actually did worse than the Xbox did over the last 2 years of their lifespans.
Also, the PS2 has been doing so well because it won the previous generation. If anyone thinks the GameCube would be doing as well as the PS2 is right now if Nintendo would have given it SSBB, SMG, and other such games, they have something else coming. The market leader always enjoys an extended lifespan, simply because it gets the majority of the games, especially in the later years of its life, making it a very nice option for those looking for a cheap system, one that has a good amount of games. The GameCube was cheap ($99 cheap) for a long time, but since it lacked the games, it didn't sell. The 360 isn't going to have a longer lifespan simply because Microsoft wants it to, and neither will the PS3.
I expect the 360 to sell for the next 4 years (end of 2012), and reach about 40 million max. You can see my (predicted) yearly sale breakdown in the spreadsheet files that you can download from my sig.
Edit: Fixed a few things...
