It doesnt "have to" but if the sensor is outspected by the 650D and/or 70D then its likely many people will buy those models instead.
Nonsense, people don't buy the 7D just for the sensor, by that logic why would anyone pay Â£1200 for the 18mpx 7D when they can get the 550D with the same 18mpx sensor for just over Â£500 !
It's like telling me to trade my 450D in for a Sony NEX5 because it's got a better sensor ! The 7D isn't a consumer camera, unlike the entry level models, it doesn't compete in the shop window on pixels per pound, number of fancy picture modes and meaningless word count on the camera body.
Someone in the market for a 7D isn't gonna cut a corner and say, damn, I'll take that fancy 22mpx Canon 650D and save the extra Â£600 ! They want a 7D for a specific job, that's better AF, faster frame rates, bigger picture buffer, built in flash transmitter, shared batteries with their 5DII, similar handling to their 5DII or any combination of he aforementioned.
I'll show my backside in Harrods window if a 7D appears this side of Xmas 2011, in fact, I'd be surprised to see a 7D before the 650D or 70D... Common sense is heading towards a 1D(s) announced in Autumn ready for Winter, a 5D & 650D in spring 2012 (Olympic big sell on the 650D), I'd suggest a 70D by autumn 2012, with a new 7D sometime in the winter 2012/13