I look forward to the day that the higher end bodies can go mirror less. Just because mirror less bodies can be made smaller, doesn't mean the have to be.
The obstacle to this would be getting contrast focus (live view) to have the speed of phase detect and of course producing an electronic view finder that pros would consider an acceptable replacement. This could be a long way off, but who knows.
Imagine getting rid of the slow and noisy mirror and never having to AFMA a lens. If they could solve the two issues above, I can't imagine anyone would resist.
I believe that ultimately, mirror less bodies will be the future.
Next stop, electronic shutter:)