Pentax are already in medium format.
No matter how cheap the sensors can get, decent glass will always be incredibly expensive, and glass is always the majority of the costs of a photographer's gear bar occasionally studio lighting.
As for 'opinions'
1. The difference in IQ is not noticeable until you're inspecting gallery prints or 100% crops. Fact. Human eyes just aren't that good.
2. The longest lens I know of in medium format (645D) is 400mm f/5.6, which is roughly 250mm FF equivalent. That's not long enough to make sports viable and the lens costs £3,000 already. So again, it's a fact that long enough lenses for wildlife and sports don't exist in the MF world.
3. It's a fact that no MF setup exists that gives shallower depth of field in the real world than an 85 at 1.2 or even 1.4. The closest you get is a Haselblad 110mm f/2 which gives a similar image to a 77mm f/1.4, which is close, but then the vast majority of everything else on MF is f/2.8 or slower, and it gets slower a lot faster as you get longer than you get on FF.
4. DXOMark tests have proven the D800 sensor to be more than capable of trouncing current MF cameras (in every area other than cropping/apparent sharpness) and even the D800's dynamic range, colour depth etc. are largely luxuries over other cameras rather than genuine advantages.
5. 35mm cameras can basically shoot in the dark already, fact. At this rate of progress the better noise performance argument for larger sensors will fade away. That's the closest thing to an opinion in the whole argument.
