What about Sony's SLT (Single Lens Translucent)? Have mirrors that don't move - so you get an optical viewfinder with no moving mirror flapping around during shots. Is this the future you think?
If the mirrors are replaced with translucent ones, there will be some loss of light (as i understand it). If the translucent mirror is fixed, the low light performance will suffer. Of course, you move the translucent mirrors when you take a photo and only keep it in place in case of movies to get around this problem.