Hi adidasss, long time!
It would definitely be sad to see all that posting history go. It's easy to migrate a database containing all threads & posts to a new forum, but indeed that would require some help from Yac / owners and - unless things have changed, I personally don't see that happening. If they were willing to get that involved with the technical side of things, they might as well update this site.
Regarding forums in general, I am a member of several and to me it seems that those which thrive the most (are most active) tend to have meaningful discussions that are on topic and make sense to outsiders. For example, posts generally add meaning to a discussion which doesn't stray off topic to be about some internal drama between members or going-ons in the community (which an outsider wouldn't "get" anyways).
It takes a lot of moderation to keep forums like that and I genuinely get that not all people enjoy that kind of moderated environment. It would actually make sense that MB today is mostly made up of people who don't like that and who are happy to stay a small, tight, internal community, even if that means there's going to be less participants over time and that the site will become less relevant in the bigger scope of online music discussions. That's a completely legit opinion to have too and I respect that.
What could still be an issue to any user is - how far can you trust the owners? They're the ones who manage all those posts and threads that we care about. Do you know they're gonna take care of it in a good way? That's the kind of question which increasingly bothered me when I was an active user here. It seemed to me like it could all be gone one day. So, I still have backups in text format of some of my posts here (reviews and guides).
It seems ironic that as a community becomes so entrenched and so tightly knit - everyone knows everyone - that they shouldn't also want ownership and more control of the very platform that enables them to get together.