Sure - but there are excellent m43 primes that are relatively inexpensive (the best and most expensive is probably the Olympus 75mm f/1.8, which costs c. $900 - but the marvelous 45mm 1.8 and 60mm macro are half that).
Considering the sensor size I'v always felt m43 primes were rather expensive as well, I'm guessing because as with Leica building that much smaller costs more.
I look forward to the disappearance of mirrors, for various reasons. Whatever else one might say about the relative performance of my Canon 5DII and 6D vs my Olympus OM-D, the latter invariably nails focus precisely (and it's nice to buy a new lens and not worry about front/back focusing, let alone trying to fix it). I don't much care whether the resulting cameras are smaller (the top of the line Panasonic m43 body isn't much smaller than a 6D); in fact, as long as I'm going to be using my Canon lenses, I would prefer the body to be bigger for the reasons given by others.
do want a viewfinder, though. In some ways electronic viewfinders are already superior to optical viewfinders. I realize it's nice to see what you're photographing as you're photographing it, and what you see through an optical viewfinder is certainly nicer (though that will doubtless change as technology improves), but optical viewfinders are misleading - it's surely more useful to see through the viewfinder roughly how the image will look as you're taking that photo that waiting until you've taken it and checking on the monitor. One of the OM-D's dials adjusts exposure, and it's nice to be able to do that while you're taking the photo, looking through the viewfinder and seeing what happens as you turn the dial.
The ideal situation would I'd say be a system like Fuji's recent cameras were you have an OVF and EVF in one finder, just have the mirror flip up at the touch of a button.
I do think we'll see a few compact FF mirrorless cameras in the coming years going after a market similar to Leica(but with AF) but I have my doubts there going to make up as much of the market as on ASPC/43 sensor sizes.
I actually think the more obvious size advantage would be when you push up to MF if theres a long term market for sensors that size. When you get to that size I think the mirrorbox clearly starts increasing the size of the camera beyond that which could be argued to be ergonomically ideal.