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 think that might happen more often on mastodon, since if you reply to a thread there, it gets boosted to all your followers.
It might be interesting to add a sorting method to Lemmy, that would bump posts every time a comment is added.
Perhaps something to turn on for each community individually ?