The release of Natal agrees with you, as does MS.
Really, unless they panic if Natal doesn't go ganbusters, MS would be crazy to release too early. Developers aren't going to embrace even higher costs which is the result of a big specification jump, plus the Wii and PS3 wouldn't suddenly die but would likely see boosts unless MS also kept the 360 on the market as Sony did with PS2.
The most likely result of an early, more expensive next Xbox would be a mass jump by the core but the loss of the momentum MS is trying to build with more casual gamers. They'd be throwing that away and probably giving Nintendo and Sony the gift of that whole market for the remainder of the Wii/PS3 lifetime.
In short, there is indeed no need for a new Xbox right now.
Try to be reasonable... its easier than you think...
