That's true but I still think that the fact some forums are paid has a lot to do with the quality of content and the downfall of many forums. People will post any random comment if they think they will get paid so the quality of the forum drops, the advertising becomes non existent and the forum fails.
If a forum is unpaid people join because they are genuinely interested in what the forum is about so will have interesting discussions. I belong to a Smart Car forum and I go there for advice because I know people know what they are talking about.
Were you talking about paid or unpaid forums?