I would gladly have 50 megapixels and possibly a medium format too.
I think 'good quality' is subjective. I can see a huge difference between the 8 megapixel and 16 megapixel shots at 13x19 inches.
For the screen viewing it doesn't matter much. Higher megapixels shots can be a bit sharper when downsized, but it's a small difference.
5 megapixels maybe enough if you view the picture from far away, but sometimes you want to be able to see fine detail:
For example, what are the people doing in this landscape:
You may have missed it, but there are people standing on the rocks, towards the right. It's very obvious on a large print, but at 5 megapixels, they would be just a few pixels across.
Or you can see the fine streaks of sand being pulled by the water (bottom left):
Each streak would again be a few pixels across on a 5 megapixels.
In short, 5 megapixels might be enough for 95% of the shots, 16 megapixels for 99% of them, but there will always be a select few shots that would benefit from higher resolution. The same goes for 400,000 iso sensitivity and all the other technical improvements.