While their might be diminishing returns in higher megapixels, i can't quite buy into lenses resolving a certain number of megapixels and no more. As said above center and corner sharpness are 2 different things. And, our current technology borrows from the neighboring pixels to obtain RGB for each pixel, when each is R, G, or B natively (all before the "RAW" image is recorded.) And, from lens to image, there is an analog to digital conversion involving optical filters on the sensor, and proprietary algorithms in the computer. I just don't see a lens being a single resolution.
In general however, obviously more and more mega pixels are going to need a better lens on a steadier platform to active the same per pixel sharpness.