Shifty, the thing to remember is that console hardware is expected to become cheaper over time, while remaining just as able to run all the games. A large portion of the market won't buy in at the starting price and thus the vendors need to lower the price through the cycle. That can't be done if they refresh to more powerful hardware.
Cheapest X360 is now just $180. If Microsoft was to phase it out at $300 and reintroduce an early X1 at $500, they would have lost at least half their sales IMHO. Without enough units out there, the software sales will suffer too. It'd be suicide.
And again, a marginal refresh wouldn't allow a sufficient jump in visuals, and would only serve to mess up the life of developers and publishers.
The time for the new consoles is now. Even just a year earlier, the hardware would have been too weak; and a year later it would've been too long for a single generation and other vendors and platforms would've become a much bigger threat too.