I always chuckle when I see folk wanting full frame mirrorless. They just haven't got the concept. Big sensor means big lenses. big. big. Not small.
I am one of these folks ;-) ... because
* it enables (r)evolutionary lens designs without retrofocus constructions
* it reduces size, mass, complexity for the mirrorbox and the penta prism
* it would help to reuse my FD lens collection with their great manual focus rings - for video e.g. - via an adaptor (if a FF mirrorless of canon hasn't a native EF mount).
But your arguments about image quality are very important: I am learning
to get out the most of the 40D sensor technically
which has just 10 Mpix and is 6 or
7 years old (in terms of technology). So: APS-C is at least comparable to the
best 35mm film cameras/films if not a lot better.
Tool-wise I am well equipped - the largest "construction site" for me ist to
get out the most of a camera system in terms of technical quality andcreativity
- I still have to improve the latter one!