Discourse is going to pull the same shenanigans as Reddit and Twitter, you can be sure of that. No one is going to host millions of users for free, forever, they'll all come around to get their investment back one day.
Yes, the respective projects themselves. If you can't trust a project not to fuck over its users, why are you so concerned about the hosting of their discussion platform in particular, rather than just about everything else?
Do you think self-hosting means "it's free to host"? Do you understand someone is paying the costs of hosting, and that someone can do whatever the hell they want to recover such costs?
usually those costs, for a specific community, are not very high. not sure about discourse specifically, but you can serve thousands of users for relatively cheaply.
https://discord.com vs https://www.discourse.org