3000+ replies going back to 2006 is TOO MANY
Perhaps a relatively simple solution would be to peel-off a calendar year's worth of replies that are older than the current year and/or the just past year (in this case 2006 and 2007 would be put in an "archive" thread) thus always keeping at least one year's worth of replies in the current thread. Then at the end of each subsequent year, the next year's worth of replies would be moved to the "archive" thread. This might keep the current thread somewhat reasonable in length and also minimize the work (possibly could even be automated) associate with the annual "house cleaning".