I've shot professionally with 1DX2 for the last 3 years, making a living shooting action sports in the Arctic; both photo and video. I also own Canon cinema gear and quite some lenses.
Adapters: Hate 'em, but will use them if absolutely needed. I'd pay another $1-2 if it would mean that I got rid of it. If something breaks on a shoot you need a replacement, which is expensive and not always easy to find. I'm hesitant to carry 3-4-5 adapters to fit all lenses.
For a 1DX3, mirrorless or not, it would first of all be as good as the mark 2, but then add something that would make it financially reasonable to buy it for. 10 bit 422 internal, 8K external, log, double battery life, 40 MP. Something in that range. I don't care about 5 more MP or 2 fps more, and I don't think any other pro does either. The camera needs to bring in jobs or take over for other existing gear to make sense.
Canon are workhorses. Yes, reliability is the main point with the 1D line to me and many of my colleagues, and I don't really care what Sony is doing until they fix sealing, battery stuff and the good ol' ergonomics.