To me, the main advantage of DX is the greater reach with super teles. E.g. I used the D500 with 500mm, 600mm a lot. That option may be an issue if one uses a mix of Canon R and M mirrorless. Canon's switch from FD to EOS/EF way back in 1987 is now distant memory, and Canon benefited a lot of that mount change. I could have never imagined that Canon would now end up with two different mounts for DX and FX mirrorless. There is no reason that Canon didn't take FX into consideration when they introduced the EOS-M a few years ago.