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