I don't think a time limit would mitigate the problem? — People won't return to vote, once the time limit has expired. They'll just forget about it. And new visitors (that arrives after the time limit has expired) will do as usual: read the topmost comments and upvote them.
If this is a big problem: I've heard people mention it, and I think I've encountered it sometimes. But I guess it is a rather serious problem, because people are so terribly lazy. I mean, they are short of time and have to prioritize.
It would mitigate the problem because then you couldn't upvote top comments period. If the assumption is true that all top comments are old, then this simple trick would definitely help.
Also, forbidding votes on top comments doesn't help a comment at position 10 or 20 to surface to the top. (So really useful comments posted rather late, would still be forgotten forever)
If this is a big problem: I've heard people mention it, and I think I've encountered it sometimes. But I guess it is a rather serious problem, because people are so terribly lazy. I mean, they are short of time and have to prioritize.