It's daunting to think of the amount of useful info that has fallen off of acmlm's boards.
That's a good example. It's hard to find anything there if it IS still on the board unless you're a regular. It's also a bit far fetched to keep a forum going indefinitely without ever pruning it even if you have the best of intentions to preserve all of the information.
Look at our smaller board. We have 35,000 posts. It's already becoming difficult if you're looking for something specific that may have been posted here early last year. Instead, in the database, there is around 2000 items much better organized than forum posts and easy to find and sort through. It's simple logic. With the database, any new person can come to the site and usually quickly find some helpful resources if they are here. With the forum, you best remember specifics about what you're looking for or know somebody who knows where it is.
Only by saving and building on information can we advance. Losing the information isn't going to help us.