But this would still result in the topmost comments getting most upvotes — all topmost comments except for number 1?

Now I'm oversimplifying, but the effect would be that comment 1 and 2 swapped place over and over again. Or the first N + 1 comments, if you restrict voting on the first N comments. (And people might be confused, perhaps annoyed, when they cannot vote on the first N comments?)

This would not help promoting [a really good but forgotten comment] that is located far away at position 10 or 20.