Hi,
My take is that if you want to make great pictures you would go with an up to date system having a good choice with excellent lenses. So, I would go with a system having a Sony sensor and than look at my needs.
I use an A7rII with Canon, Contax and Sony lenses. That works for me as I shoot on tripod and focus manually. Canon has some great lenses, but they lag a bit in dynamic range at low ISO.
With the A7rII you can use almost any lens ever made on the planet, except Leica M lenses. But Leica M lenses were not made for digital sensors, so using Leica M on digital is a compromise. Leica solves some of those issues with ultra thin cover glass and sensors designed to support M lenses, but that is still a compromise.
Best regards
Erik
Ps. Hasselblad, Phase One, Pentax, Fuji, Nikon all use Sony sensors. Canon and Leica have their own designs.
Should I get a Leica or GFX... ?