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).
I’m not against necrobumping in certain contexts. If I have a tech support question, I can promise you I’m still clueless after 5 months. Also a niche creative project can take over 5 months to find it’s audience, and creators are usually happy to hear feedback.