I believe they postpone progress as much as they can to milk as much money as possible (Just my opinion, I cannot have proof of course).
Mmmh, I don't think milking customers to hold back on technology is such a big issue here. In other industries I see this as a valid strategy, but not in our current camera market. There is just not enough pressure on R&D to make bigger leaps. That's all.
We can thank Canon, Sony and Nikon for their competition, because this is what primarily drives this market forward. It would actually be dreadful for Canon or one of the others to provide a camera, which is so good, that nobody would want to upgrade in the next decades. Just look at Leica as an example on how to do things wrong.