I’d imagine that charging ~$3/month would rapidly improve the quality of discourse on most large online forums. There are benefits from network effects but considering the resiliency of club goods like universities, suggests to me that we should see a lot more online club goods.
And yet Reddit, Twitter, YouTube, Facebook are all free, and there are no apparent competitors that charge for membership. Even television, which has made the jump from cable to internet, is returning to packages at a fee. Why not online forums? Hypothetically, how much better would an online forum have to be than its corresponding free option in order for you to consider paying for it?