I'm not sure if this is what you meant, but a blockchain doesn't require that every user runs a node. And any "blockchain" application usually involves some off-chain brokers for performance reasons, which yes users would have to trust to some extent, but they'd also be able to cross-check and switch brokers (or run their own) if they ever lose trust.

So a blockchain forum is doable in a similar way as NFTs. Would require caching on the read side, and unless your users are posting important enough stuff to warrant high xact fees, off-chain write batching.

The real obstacle is that like NFTs, it'd be solving a mostly theoretical problem. Barely anyone cares that a forum is owned by someone, and you need a ton of users to gain traction.