these things are so good nowadays, progress is going to be smaller and smaller from model to model IMO. I also doubt that Canon builds these for those that have Must Always Upgrade Syndrome (MAUS), I'd think that segment of buyers is rather small, as we're not talking $200 phones here. I would guess they target new buyers and those upgrading from much lower grade or several generation old cameras. Maybe I'm wrong, but my guess is those that had the 10, 20, 30, 40, 50, and 60D and now order the 70D are a very small minority.
Now they're adding better movie things, wifi, touch screen, because much much better sensors are probably way too expensive to produce for a big market. Most people don't have a couple thousand to spend every year or other year on upgrading, most don't need to be the first on the block to have the new toy. Not in this price class.
I have my 40D and just got a 5D3. Eventually I'll get an other crop and might give the 40 to my boy or have it changed to infrared. I never felt the need to upgrade the 40D, love that camera and it works perfectly well. I did want full frame for other reasons and wasn't impressed by the 5D2, so I waited a bit longer and am very happy with the 3 now. I sure won't be upgrading to the 5D4 unless it also makes coffee and cleans the house. I'd rather save a couple years and maybe get a medium format for studio work.
And I never pay attention to what Nikon has, I don't care. I have canon gear and unless they fold or create paper cameras I'll stick with what I have. I'm sadly not made of money, LOL