Most people are only willing to pay with non-monetary resources (PII, ad data, etc.). You can't approach this with charging money in mind, because people will just go back to the places where they aren't expected to pay. Start charging for Mastodon? The majority will go to Bluesky, Twitter, and Threads. Lemmy would just feed back to Reddit. Either that or they'll drop off social media altogether.
We've already got proof of this: PeerTube. Most PeerTube instances either charge a fee to upload (call it a 'donation' if you prefer, but if you're gating an action behind money, that's a fee), or simply don't allow any users not connected to the admin to upload. YouTube, Twitch, Dailymotion, and a few other sites are free. The sites where it's free to perform the core activity will keep winning, especially as we see rising inflation and increasing costs.