I think they already rate limit down votes. I think I recall seeing at least a few not apply when I checked as an experiment in the past.

I try to up vote at least twice as much as I down vote. If I down vote a a low quality comment, I try to at least up vote 2 quality ones to offset it.

Slashdot had (still has as far as I know) a daily limit on up and down votes, and quite a small one at that (I think 5?). It also prevents you from voting and commenting on the same topic.

Or just make it cost karma. Anyone that down-votes too much would wind up at the threshold for down-voting and be rate limited by their ability to make up-voted comments. It would also render ineffective the strategy of having a bunch of accounts with enough karma to downvote and using those accounts all at once to make comments you don't like dead (the presence of corporate or state controlled accounts employing this strategy has been suggested multiple times).

The way to game that is to just upvote random comments that you don't care about in stories that you don't care about, just to have made enough upvotes to enable you to downvote again.

I meant have comments that get upvoted, not upvote other comments.

Oh. That's harder, but not much. You just need two accounts. Make some innocuous comments on account 1, upvote them with account 2. Now you can downvote from account 1.