I know and understand image resolution is a result of system resolution, I just pointed out, with images, the system resolution of an 18MP crop camera is not very much different from a crop from a 21MP FF camera. Again, that is not an argument, it is an empirical observation.
Maybe I'm oversimplifying the original question--"the benefit of megapixels", but all things being equal, more = more. The rest of this is arguing that all things aren't equal, which is obviously true most of the time. A 16MP 1Dx just might
take a better picture than Nokia's 41 Mp cell phone camera cropped to 16. All megapickles are not created equal, but that does not negate the fact that more of "the same" MPs means more freedom for cropping. If you don't believe me, shoot an image in RAW, then the same image in sRaw, crop a chunk out and see how they look.