But you're not, as you say, "eliminating the mirror". You are using the sensor as the mirror.
I see several major issues:
- A sensor isn't nearly as reflective as a mirror is. The mirror reflects close to 99% of all light that hits it. The viewfinder would be, at the very least, extremely dark.
- Live view would not work
- moving the sensor around very fast, and just before exposure is fraught with problems. The mirror can move quickly because it's light, AND the only thing you care about is that it is out of the light path. Moving the sensor to vertical requires very precise positioning, and no bouncing.
- Leaf shutters are used in medium format SLRs, but there are limitations to leaf shutter construction. I'm not sure speeds of 1/8000 are practical in a leaf shutter. Add to that, the leaf has to be open for focus and metering, then shut, then opened & shut, then opened again. Sports photogs and wildlife photogs would probably not be too interested...
- The DPAF wouldn't work properly. With the sensor/mirror at a 45 degree angle, correct focus is different between the top of the sensor and the bottom.
The mirror allows a lot of light to the autofocus points that are behind it, so not all of it is reflected.
The sensor reflects quite a bit of light (I don't know if enough), a nano coating could be optimized to increase the reflectivity at 45 degrees.
Live view would work just like now, the sensor would go into vertical position.
The sensor would not be attached at the edge, but it would be through the middle, it would pivot on this axis. It would move half the distance that the mirror does now.
Vibration seems to be the biggest problem to me.