That isn't the issue at all. It's the reverse.
Think about at the moment. You get an 80D and you buy some lenses. Let's look at someone who bought an EF-S 18-55, an EF-S 10-18, a 50mm f/1.4 and a 70-300 IS non-L zoom. Maybe your later lens choices were in part driven by knowing the lenses are future-proof if you decide to switch to FF.
Part of the incentive in upgrading from the 80D to full frame is that at least some of your lenses can continue to be used.
Now, if Canon had for example kept to the EF-M mount for full frame, in the future we could have the same situation, low cost 50mm f/1.8 or 1.4 EF-M lenses and a low cost 70-300 that would work with the EF-M but would also provide a full-frame image circle for when you upgraded to a FF camera.
Instead, you're stuck with APS-C only EF-M lenses, and the incentive to upgrade is far less. Suddenly the entry cost to FF is prohibitive.
This doens't affect Nikon because they have no APS-C mirrorless, and it doesn't affect Sony because they share the same mount.
Now, the advantages of the new mount are almost certainly worth the pain, but it is now going to be difficult to justify recommending EOS-M cameras to anyone because they will be stuck in an APS-C forever ecosystem.