well, i missed that thread when it happened, but i just wanted to underline that "old" forums, especially those focussed on a specialty topic, like the former monome/community should not ever be deleted.
There is an invaluable amount of information and knowledge in there, contributed by many people. Google search often directs me there as a matter of fact, and it has more than once been to dig up some "ta-dah"-inducing info.
I have in mind two forums that revolved around the broadcast industry, that have gone in a blackhole a few years ago. Those forums were gathering total noobs, amateurs, seasoned professionals, and engineers of world class manufacturers and media networks. They were cluttered, but well indexed and the mountains of knowledge in there helped me become a better professional in my field. Reference books are one thing, but the shared experience, the tiny details, are what really makes a difference.
When a forum goes away, also goes with it the willfullness to contribute of people who invested time in sharing, and the possibility for newcomers to better apprehend a field and start on the shoulders of those who came before.
As for how to archive, given the context i would think that converting the whole thing into static files is the simplest and most future-proof way, as it "only" requires to maintain a simple file server. No idea how much space it would take though. (I just launched an httrack process, so i guess i will know in a few hours/days.)