I disagree with the thought of the crop sensor cameras going away any time soon.
10 year flashback - remember all those folks who said film cameras would never go away? Granted, they're not completely gone, yet. But then, there are still some people using rotary phones, and I'm sure you can find a working teletype or two somewhere in the world.
When (becuase it's not 'if' it's 'when') full frame sensor cameras drop down into the sub-$1000 range and thus into the mainstream consumer price range, APS-C will die off. EF-S lenses? They'll just join FD's on the auction block.
While I would wish and hope that full frame sensors become the norm and drop sub $1000, i just have the feeling that wont be for another 5-10 years... The cheapest full frame camera now is what? $2500? Well, correction, NEW full frame camera disregarding the used market. With the increased MP and technology going into these sensors, I would think that they would have to either dumb down a full frame sensor to go into a rebel or really cripple the featureset more than it already is to afford to justify putting a full frame sensor in... perhaps full auto with just a shutter button? All kidding aside I cant see that happening. I could see the 7D and xxD series getting the change before entry level... at least they could then justify price increases in market prices, but until then...