I’d argue XMPP is less ideal than Matrix because groups are located on a single server, which makes them easier to take down than Matrix’ replicated state.
Running any P2P/decentralized protocol over I2P seems to be the best for privacy and censorship-resistance. I2P already works great for torrents, except for it’s speed and lack of users/seeders.
The problem always comes down to usability and barrier to entry. Telegram is popular because it’s great to use, and doesn’t moderate much. More private services rarely (never?) reach the level of usability most people expect, often simply because of it’s architecture.