I think there should be some incentive for that, like make those kinds of comments a spotlight or something. Maybe make a community called “late replies” that showcases the best such replies, or have a rule saying they grant free karma (in Reddit’s case).
Within the last year, I’ve definitely had some replies to old threads of mine. And if it’s a question, I’ll respond. And I think I did comment on another’s thread at least once, confirming and thanking them for the solution. All of these were tech support related. I feel like that’s one of the few topical areas where that’s acceptable.