Andrew Bossom suggests a work-around that lets you respond to at least some of those old comments, and hopefully will let you get the discussion going again on your channel. If the channel that posted the original comment is linked to Google+, you can post a new comment that + mentions the commenter. They will receive a notification of that new comment that mentions them.
Of course that only works with channels that are connected to a Google+ Page or Profile. If the channel is not connected to Google+, and it's important to reply to them, the only other option I can think of is sending them a private message. At least then they won't think you've forgotten them.