I'm not sure Nikon or Sony care at all. It is not hard to make a medium format sized CMOS, the real issue is the cost which tends to increased quadratic ally with with the area (a sensor twice as big, e.g. a FF compared to APS-c costs 4x the price). Afaik there is no cost difference between a CMOS and a CCDS sensor, the cost is proportional to the surface area.
Neither Nikon or Sony have any MF lenses so they would have to start a whole new line which is costly. And the way things seem to be moving is to smaller more compact systems, not bulkier bodies with even bigger lenses. Saying that Sony is desperate to try anything once to see what sticks so they might do something unexpected. But note Pentax has had a perfectly good digital medium format camera with a large lens selection and that hasn't developed any significant market share.
Edit: Would be nice to have an affordable MFD cam but we are a long way away from these being prosumer costs.
