This would prevent karma gain from "interest" on old comments

If you want to reward having a long history of comments, you could prohibit only downvoting of old comments.

it wouldn't prevent ongoing retributive downvoting

I doubt you could algorithmically distinguish between downvoting a horticulture post because of disagreements about horticulture and downvoting a horticulture post because of disagreements about some other topic.

But I suspect voting rate limiters should keep the problem in check.