Henry, with the tech we have today, there is no reason that a company can't supply a basic DSLR with a modular mount. We have adapters for just about every AF mount to either m4/3 or E-mount (with AF), and every conceivable MF mount, so we know it can be done from an engineering standpoint. I'm not talking adapters per se, a semi-permanent (but changeable) bolted on mount.
Image being able to buy your favorite camera - be it A99 or D810 - and specify "I want that in a Canon mount." Then, when you want to give it to your brother-in-law who has Pentax, he would just take it to a camera shop where a new would be installed and maybe firmware updated. Modern PLCs make that fairly easy.
The problem is, all of our current camera companies - except the 4/3 group - are too much into selling lenses and system accessories. I have seen a Kodak DSLR in Nikon FA mount, but no of no one outside of the 4/3 group that make anything except their own mounts.
For such a camera to go over well, you would probably have to have a modular flash shoe as well.
Such a thing would surely be disruptive, but I think would go over well. I could then but a new Pentax FF that's coming out for under $1000 with Alpha mount and not have to lose IBIS or sell off present equipment to get what I want - a new OFV DSLR with IBIS in Alpha mount.
A couple of a350's, an a700 and now an a580, plus even more lenses.