The issue isn't that content rises which isn't good, it's that good content doesn't rise. So there might be 3x more good stuff getting posted than actually makes it onto the frontpage.
Yes, thinking a little bit more, I agree with you. But in my case, just as it is, HN has so much good content that I can't read it all.
I already can't read all the good stuff I'm seeing on the front page...