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 commented on a 9 y/o Reddit thread yesterday
It almost time to celebrate the 12th birthday of “can splunk stop being absolutely fucking clueless and claiming their stupidity is a legal problem only they suffer” ticket about authenticated yum repos.
Usually I find my own answer to a Stack Overflow problem I’m having today, but I wrote it two years ago. And that means I’ll have that time machine one day. Woot!