How could Canon improve the 600D without competing the 60D??? And how can they improve the 60D without competing the 7D...
Remember earlier this year, when the T2i/550D already had the same sensor as the 7D and 60D, and lots of people wondered how the next entry level body could be released without cutting into 60D territory? Yet we got a 600D/T3i anyway, right on schedule.
As we saw, there don't have to be big changes in that line. The idea isn't really to induce current 600D owners to upgrade, it's mainly to keep the consumer line 'fresh'. For example, the xxxD line has used the same AF sensor for a long time. Canon could go from 1 to 5 cross-type points (still 9 total), allowing them to maintain separation from the 60D while touting a 'substantially improved AF system' for the T4i/650D. The 70D (because the 60D still uses the 40D's AF) would get, say 13 points, and the 7DII would stay the same (oh, sorry...Canon will say 'improved AF algorithms supported by the new Digic V). That's just one possible roadmap, off the top of my head.
I know *a lot* of people that haven't bought the 7D because the 60D was close enough and "the sensor is the same anyway". I also know an awful lot of people who didn't go xxD because the xxxDs are pretty damned good now (especially the T3i) feature wise.
How many is that? The thing is, Canon 'knows' *ALL* the people who have bought their cameras, and sales of the 7D and 60D remain very strong...so unless your 'a lot' numbers in the thousands (or tens of thousands?), they don't seem to be affecting Canon's bottom line.