You're describing a federation of platforms, and this can work, at least technically. XMPP worked wonderfully for a brief moment. Unfortunately, profit incentives are aligned against it.
The Fediverse, at least as I see it advocated here, is not a federation of platforms but a federation of social cliques, where your ability to communicate with someone else is determined by connectivity in the clique graph.
This is a great thing for the kinds of people who would have become deeply invested in a web forum or newsgroup back in the day, but it cannot (and arguably should not attempt to) address the mass market.
The Fediverse, at least as I see it advocated here, is not a federation of platforms but a federation of social cliques, where your ability to communicate with someone else is determined by connectivity in the clique graph.
This is a great thing for the kinds of people who would have become deeply invested in a web forum or newsgroup back in the day, but it cannot (and arguably should not attempt to) address the mass market.