Without any intention of be a know it all (because of course I am not privy with the details): from a purely technical standpoint, have you considered checking the technical details with someone?
Because 616.472 posts and 57.317 topics is not that much amount of data at all. Even assuming a hefty 100K for each post (which I seriously doubt), the grand total would be.. 61 Gb. The forum does not host memory consuming data such as images or video, so that would hundred kilobytes of basically only text - an enormous amount of writing.. Even considering UTF-16, so maybe 2 bytes per character, 50K of text is perhaps the equivalent of one SOS issue's worth of text - for every single post . So the actual db is likely much smaller. Or did u mean 616 million posts?
It's actually (with no disrespect) laughably small in data bases terms.
Even ten times that amount shouldn't impact performance in absolutely anything, so long things are designed in a minimally ok way.
The main bottleneck to data processing performance nowadays is generally network bandwidth between collaborating machines... but then we are talking petabytes of data - hundreds of thousands of millions etc of SOS forums - with concurrent access from hundreds of thousands of users.
The cost of a small review (perhaps paid only if the review yield actionable results) could well be much lower than pruning the forum and creating and maintaining a proper auto-prune mechanism.
As a forum user, anything that works for you guys works for me - SOS forum is one of the best free resources of the internet so you're always right. So let's just say I have put on here my old hat of consultant - for once, at no cost ;-)