The problem with basic protocols is they are inherently very slow to develop and iterate, so problems stick around for a long time. UX can vary widely across different clients for the protocol. SMS has had huge longevity and will stick around for a good while longer, but I don't think anyone is arguing that SMS is actually better than the internet based closed systems that exist today.
But to answer your question, plenty and not sure if they are willing to pay. Many open source projects now a days start on discord and then eventually add a subreddit. For example, deno.
Have you heard of Matrix? Because it sounds like you're describing Matrix almost perfectly.
People don't want to "switch". They don't want to host software. They don't want to register for a bunch of different communities.
They just want to click "Join" and build relationships. Or click "Create" to create one. You can't just try to solve chat and protocols (IRCv3), you need to solve community.
I certainly agree with what you're saying. But so far our attempts at decentralized social systems are clearly early on in the experimentation process, to put it the nicest way possible. Open protocols are a great idea, but that is the easy part that we like to fixate on. The hard part is a compelling experience that people will want to use so that you have people to chat with.
And so far we're still trying to figure out what that solution looks like.