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The longer lenses get, the less difference it makes in weight/price whether it is APS-C or FF. 
Canon has enough tele lenses, it just lacks lower priced ones (like Nikon has been introducing).

But most serious photographers do gravitate towards FF... So the incentive to make relatively expensive tele options seems to lay with the manufacturers who have no FF to offer (Olympus, Fuji).

The longer the lens, the more interesting a smaller format becomes due to the crop factor. Hence, at equivalent focal length the smaller format is much smaller. I have yet to see a FF equivalent to the compact Panasonic 35-100 f2.8 or 35-100 f4-5.6. Such FF lenses don't exist (70-200 f5.6 or 70-200 f8-11.2).
As far as gravitating towards FF, this is purely a manufacturer decision, not a customers one.

You answered your own question.
APS-C public often just gets a "kit lens" or two. That is its main public. The more serious/professional/artistic crowd spends money on specialized lenses and FF, mainly.

So yeah, there still is a place for EOS M.. The APS-C public which gets a kit lens and 1 or 2 other lenses (an UWA zoom, or the "street photography" 22mm, or a 32mm f1.4 "fast normal". or 55-200mm).

I didn't answer my own question, I merely gave examples debunking Joju's claim "Given the small differences in size and costs, I really want to know why going on with APS-C."

I strongly believe that a lot of people go into FF for the status symbol it brings, not because they truly need it or for the "artistic" value it may provide in some specific circumstances (for the same reason many MF shooters claim they need MF over FF).

There is no reason why FF is the sensor size of choice. It's arbitrary and only stems from history.

You haven't seen a FF 70-200mm f5.6 because there already are 70-200mm f4 and 70-300mm f4-5.6 lenses.
A 70-200mm f8-11 is just as silly as a MFT equivalent, and won't sell. Might as well just get a compact powerzoom camera.