People love to complain about giving up money when they don't understand the value they'll get from something. Make it clear what you're selling and why it's worth $10 / month, and people will pay you. It's not as big of a problem as you think.

We can still create a world of free products that enable what your forum has -- it just needs a simple business model like, Hey, do you enjoy writing here? Send me $5! Then subsidize your free users with paid ones. If you have a day job, you can consider it a success when the servers are paid for every month.

There is nothing inherent about the internet that says it must be run on ads. That's just what ad-powered platforms want you to think, to justify their own terrible existence. Get creative, put on the tiniest of marketing hats, and just ask people. There is no quick and easy lunch, but I think you'll be surprised by what you find.