That's probably why there's not a single decentralized forum yet. And without centralized server, everyone will need to keep the entire database of all posts. It won't run in a browser, so no advertising for SEO either. Search engines don't visit decentralized forums.
Theymos wrote about the possibility of a decentralized moderation system, kinda like the current Trust system. So if you trust me, you won't see posts I've deleted. But if someone creates a billion spam accounts, that's still going to be a problem.
I expect to be stuck with a centralized forum for a very long time.
Everytime I'm reminded of this, I'm always thankful to the people and organizations that are running those servers for free or at a really low charge just so we can establish a server that's outside of what the common Internet users go to.
For free? Nothing is free here. This forums runs in an expensive server.
Also, This forum is very profitable, a lot of people is making a lot of money here.
In other forums as well. Quora, reddit, etc even pay their top contributors to post.