Torrents with Magnet links are as decentralized as it gets: one link, that can be shared on different websites and any other way you want, contains all the information you need to connect to multiple different trackers. The trackers will connect you to other people sharing and downloading the files you want. I don't think there's a limit to the number of trackers, seeders and leechers, so it's pretty decentralized.
The problems become evident when older, less popular torrents no longer have seeders. I've seen torrents with many incomplete leechers and no full seeders. If that happens to your decentralized forum, it basically ends.