It's the sign of a maturing market / product.
Was the 550D a massive jump up from the 500D? For me, yes, specifically from a video point of view, very practical upgrades. Do most rebel users care about video fps? Video manual controls etc? But evolutionary in a wider scale I think the 500D was actually a bigger step forward than the 550D, although I never ever considered buying a 500D (I actually bought the first EOS to do 25fps PAL video - the 7D)
D30 to D60, big changes. D60 to 10D, big changes. Incremental since.
5D to 5D2, big changes. 5D2 to 5D3, big changes. Frustrating to say, the 5D4 or 5Dx may be incremental.
1Ds to 1Ds2 big changes. 1D2 to 1Ds3, incremental. The 1DX maybe the exception that proves the rule.
1D to 1D2, big changes. 1D2 to 1D2n, incremental. 1D3 - lets not go there. 1D4 - incremental. 1DX maybe the exception that proves the rule.