While asking your customers to change to a new system is a big deal (and it's not guaranteed it would work as well as the last time), I don't believe in an EF-mount MILC line.
If they'll reach a point where they would want their full line to be (also) MILC, including pro cameras designed to be used with monster lenses like a 600mm, the solution will be a short registration mount; and compatibility with EF lenses will be offered by adapters. Provided that the EF-M mount is adequate, they could make a camera as large and heavy as they wish, of course we're not talking about the EOS M here.
At that point - and I'm speaking hypothetical here - they'll likely decide to give up on DSLRs, even the pro ones, and go full MILC; or at least foresee this possibility. Keeping the EF mount would be disadvantageous in the long run and for new customers; there can be no upgrade path whatsoever from entry level MILC (EF-M) to an advanced EF MILC.
In the case of a hypothetical 645 MILC, we're talking about some 5cm, which could be a good reason to do it.
Anyway, that's how I think.
On a personal note, I found a high quality OVF to be much better than those overly bright EVFs.