It's not the technology design that's that important. It's the network effect, and peoples' default trust in megacorps over volunteer projects. Both of which cannot be solved with just a protocol.

Half the problem is "felony contempt of business model", where the legal system is wielded against anyone who would create pro-consumer tools.

This would be a natural role for the Post Office to take on, to provide a neutral ad-free, privacy-respecting messaging platform accessible to all.

I think the natural and equivalent role of the USPS would be an ISP, rather than a "messaging platform" itself.

When the US Constitution was drafted in 1787, authorizing the new Federal government to run a postal service, carrying letters and packages via horse rider/wagons was the state-of-the-art.