The other side of that is that people who are willing to pay the cost (time, dollars, etc.) to run a forum are generally going to be more committed to that comm

The other side of that is that people who are willing to pay the cost (time, dollars, etc.) to run a forum are generally going to be more committed to that community. It may be more fun to run a subreddit, but subreddits are also less coherent communities than forums.