I think the last article I read stated 46 mp was needed to match the quality of 35mm.
I have to make an honest statement here. I don't think digital sensors will ever equal film. Especially nothing like Medium format. They may come close to 35mm.
At the end of the day, if you are a good photographer, what you see in the frame hits the film and thats what you get. There is no interpretation of color, what hits the film is what the camera saw. There are no blotchy solid color areas, noise nonsense, jagged edges, mistaken white balance, or shutter lag to mar an image.
There is something visceral about knowing that when the shutter of a film SLR clicks you just saved a moment in time. On a digital, you made a computerized rendition of that moment.
Some of the most famous photos on earth even taken with film SLR's could be shot 10,000 times with a digital and I guarantee you'd never get the same results.
Does that mean I give up on digital cameras? Hardly. Being able to take 2gb of pics per "roll" is ridiculous. And they will only get better. Just saying that there will always be a market for a film camera.
And I don't believe I've ever seen a film photograph with "purple fringing." (That wasn't done on purpose)