Canon's goal is not convert people from APS-C to full frame, Canon wants to sell enthusiasts two cameras, one full frame and one APS-C.
I believe you're right in theory, but how is that feasible?
I believe that for most people a 5D3 is a more sensible investment than, say, a 6D + 70D. In fact, if the 6D had the same AF as the 70D, maybe people would have dropped their 5D3 en masse, as it happened with D800/D600 when many people realized that 24 MP and 39 AF points were good enough and worth more money in their pockets.
At this point, the 6D2 will probably have at least the AF system of the 70D, and for most people that would be a deal worth throwing out their APS-C. Simply because at that point, you'd rather spend that 1-2 grands on a good tele instead of a pro crop body. Who would spend more money on a crop? Some for sure, but maybe not enough.
It comes a time when things get good enough and expensive yet marginal upgrades become inconvenient. The 7D/70D AF is not the best out there, but I believe it satisfies the most. So in this respect, if the price of such a crop body goes too high, it might lose the competition with entry-level FF, even if specs are better.
The real IQ upgrade at that point becomes what Canon seems most reluctant to provide: a new sensor tech.