Until now, I thhought 20-25mp is perfect for 35mm format. Any more and quality suffers. But maybe that sweet spot for packing pixels on the chip, with about 6 micron sized pixels, is not the plateau I expected it to be. Can a 50mp canon be good? Will it be as good as my hasselblad? Better?
I think we are getting into the territory of diminishing gains. As you increase sensor resolution, you need glass that can cope with the sensor. At the same time, you become more and more limited by diffraction. There will be a point where there is no alternative but to go to medium format if you want better quality.
How many MP is enough? That is a question for each individual photographer, based on their needs. There are a lot of times people shoot in the lower resolutions offered by cameras (SRAW/MRAW etc.) - because the resolution is enough for the use case at hand. - If you only need a 11x14 print, 10MP is enough. If you are only shooting for the web, even less will do.
Provided you can get lenses that can handle the resolution at a reasonable price, 50MP may be reasonable. (The pixel density of the 7D is roughly equivalent to 46MP on full frame.) A 35mm lens that can cope with that kind of resolution is still much cheaper than an equivalent quality lens for a medium format system. However, your diffraction-limited aperture is going to be getting down to somewhere in the region of f/6.3, I suspect. That raises the question of how much headroom you have until diffraction kills the benefit of any more MP?