fullstop said:
the mirror will go. it was only there because prior to digital imaging sensors and electronic viewfinders it was the inly way to get "through the lens" viewfinding, focussing and metering. that was the only reason to exist for moving mirrors and all related disadvantages from slap to size to alignment issues.
once the mirror is removed, there is no reason to keep making cameras any larger than necessary just to preserve a legacy lens mount, when legacy lenses can be used with a simple little adaptor.
"Necessary" being the operative word.
If you assume the target user base for your first FF-ML is the 5D type folks, the expectation is they will want to be using the glass most common to that body type. Which I believe translates into the kit lens (24-105 f4), the 70-200 f4, and the 17-40 f4. So those actual lenses dictate the user experience that first adopters will have, since it is unlikely they will release equivalent 'new mount' lenses at the same time - and even if they did, the cost would hurt sales tremendously.
So if it is a new mount, it will need to come with a really good adapter, so your first adopters can enjoy the new experience with their existing base of lens.
Now, in order to achieve the same level of ergonomics (which is likely critical to pleasing your first adopters) you have with a 5D class camera, your size will likely be no smaller than the M5, which is right at the border of too small for many folks in the western world. Hang a 70-200 on it, and it really is marginal. With an adapter stuck in the middle, even more so.
At the same time, you have to dedicate sufficient volume to heat sinks, electronics, and batteries.
All of which suggests that your first real foray into FF-ML is going to be the same size or larger than the M5, at which point it makes little sense to go for a new mount when a product using the existing one will please the majority of potential first adopters.
The SL-1 apparently doesn't outsell similarly priced T7i et al, which it should have if size mattered all that much.
At least that is my take on product development.