it has more to do with pixel density. Higher megapixel crop sensors will reveal the weaknesses of the lens.
Yes, I appreciate that, and as you were typing that, I was making just that point, but that it isn't that simple. You see the lens as "softer" because you are blowing up the image more, viewing it at 100% on a monitor. Make identical sized prints (or final jpegs) from a low pixel density crop body and a high pixel density crop body and you won't see the difference, as both images are shown the same size. View both original images at 100% on a monitor though, and you do see a difference as the higher density image is way bigger.
I am not saying that you cannot see a difference, but it only shows when pixel peeping. All you need do is tell yourself that the difference isn't "real" it is simply that you are peering closer at the image. That point often escapes people when the images they look at appear to be the size of their monitor, but in reality one is over 5 feet wide while the other is maybe 4 feet wide.