Honestly, I don't think there is much to worry about. Yes, the forum could go bust at any time.
I don't know the ins and outs and the specifics of how it works from Spike's end, and I haven't been here that long. But from what I have seen of the user base here (The few assholes not withstanding) it's a pretty tight-knit place.
Almost like a family, honestly. I don't think if the site were in dire straits, the users who care would let it sink. The backups be damned, I think most would respond to the call to keep the forum going if nothing else