You are mistaken. The eos-m mount can ühysically not handle an ff image circle. Sony was smarzer and made their e-mount just barely large enough to also handle ff. Canon was stupid, as so often. Aps-c only. Dead-slow AF. Same tired old 18 MP sensor, a dinosaur from 5 years ago. Bad low iso performance. Poor hi-iso performance.
Sony was smarter?? They now have to support both the E-mount and Alpha-mount in both APS-C and FF versions and they don't seem to be providing clear guidance about which mounts and configurations will be emphasized, thereby creating confusion among potential buyers. Fuji and the micro-4/3 manufacturers are doing a much better job of creating workable systems.
FF is the Holy Grail only because it is the same size as a standard slide or negative, resulting in an enormous body of legacy lenses. In a similar fashion, in a similar fashion, Super-35 is a video standard because of the dominance of that image size for movies and the resulting development of now-highly prized and extremely expensive PL mount lenses for that image size.
There's no technical reason why Canon couldn't release an EOS FF mirrorless system by simply shortening the distance between sensor and lens flange to 18mm, introducing an adapter allowing use of DSLR lenses on the mirrorless bodies and taking their time introducing FF mirrorless lenses. About the only thing that Canon has done right with EOS-M is restrict it to APS-C-sized sensors. Canon has publicly stated that the emphasis of the EOS-M system would be small size. It's too bad that their implementation sucks.