Optics don't know about pixels. To a first approximation, they are continuous. Whatever discretization happens after that, it's not relevant to the optics in any way. A sharp lens can greatly outresolve a low-resolution sensor; the bottleneck is the sensor in that case. Conversely, a high-resolution sensor may not be of much use if the lens is soft - but in this case you can at least in theory use deconvolution to recover detail if you know a good approximation of the PSF of the lens. This is not uncommon when working with scientific instruments, actually, but pretty rare in the case of consumer photography equipment.