There's no reason why there can't be a body with a built-in EVF and one without.
If Canon (or Canon USA, at least) is having trouble pushing one model, how will they feel about two? The other problem I see with that is that I'm afraid the two models will have different sensors - a cheap one (T5i/SL1 sensor, no EVF) and a more expensive one (70D sensor, EVF, and bigger).
I don't think the M system can survive with only one camera model. The implication would be that Canon isn't really serious about this system and Canon needs to start treating it like a system, even if a small one. As for using separate sensors for the two models, I agree with those who believe that the 70D sensor (or a close relative) is the minimum that could reasonably be put into any new M body. If not, why not just keep selling the current M body at the current price (or perhaps $100 higher)? That being said, one thing that has always puzzled me is why Canon went to the expense of revising the firmware in a model that was going to be replaced in a few months.