Four FF bodies in the current line up (if you include the 5D mk II), and four different sensors (22MP, 21MP, 20MP and 18MP). Six crop bodies in the current line up (7D, 60D, 650D, 600D, 1100D, EOS M), just two sensors - 18MP and 12MP - if you count the with PDAF and without PDAF versions of that 18MP sensor as one.
I can't see any reason to carry on with using just one sensor across the range other than entry level - why not develop more than one, like they have in their FF cameras? They sell so many more crop bodies than FF, you'd have thought if they were going to share tech anywhere it would be in the FF range, not crop.
If Canon start to show the same level of R&D in crop sensors as they do in FF, there's no reason why the 70D couldn't come with a new sensor, followed by the 7D with yet another.