There’s a pretty good argument for this now. Years ago, people would never even THINK of paying for social media. But now we see the consequences of getting things for free. People have no say in what happens on Facebook or Twitter— ad dollars and sensationalism do. Do you think they would ban and censor so much if they knew their user base could rip away their funding?
A small paywall would also prevent spam and bots from trashing the place, and would likely also keep people from flooding over to hijack the platform and drown out the real community.
Additionally if enough people subscribed, it could prove to be a much more effective way of improving the site rather than relying on Patreon.