Actually the more time that passes the worse it is. For example, if we lost this forum today, we'd lose a week or two's worth instead of losing a decade's worth.
It might make sense to have a plan for archiving to a simple, stable form ahead of time, as you go. That way, when the forum eventually dies again, as all things do, a stable read-only archive of the whole thing could already exist. As it is, I manually copy bits I care about because I've been burned by dying forums too many times (not only this one.)