Back when I was a regular on LessWrong and it had an active community, it didn't have that problem at all, despite using a variant of the Reddit codebase. Their solution was pretty simple, just show the five most recent comments in the right hand column. People would interact with them, check out their parent comments, and threads would grow organically regardless of age. In fact people would often come back to very old threads and revive them. It might be tricky to scale that to larger communities though.