But how big can torrents be allowed to be?
You could store all subforums separately, then it will be much smaller already. And you could create yearly/monthly snapshots with only the new topics. In general I'm quite fond of the torrent/IPFS idea. It will also show which subforum is really the most popular, and which ones would see almost no interest in storing or even downloading it.
* To know which sections are legitimately "newly stored" could be a quite difficult challenge. It maybe is impossible - at least without the forum operators themselves acting like a sort of "oracle" confirming the new threads are legit. Or there could be PoS voting (a kind of DAO) which new parts are "accepted".
The way the site's content is stored can be a problem. But the even bigger problem is making the site functional for anyone. All information must be accessible within seconds, for anyone who enters the site. No need for downloads or other extra programs to access.
Furthermore, all information must be 100% online, and it is not possible for someone who has certain content on their PC to go offline, meaning there is no longer access to the information.
This is not simply a question of storage, but of the entire functioning of the forum.