I agree what you described - one client getting really popular is a huge risk, but the risk is not the popularity of one client, it’s the popularity of the client without any user knowledge of the underlying protocol. I recently wrote about XMPP and this problem https://salibra.com/p/the-death-of-jabber-8ea395e82f5b

The risk you’ve described isn’t as insurmountable: So far, bitcoin, ethereum, & BitTorrent serve as strong counter examples to the problem you state. They’ve remained clearly defined protocols that users seek out instead of a particular client and have proven hard for popular clients to fork.