I think the network effects make email hard to replace, virtually everyone online has an email address. If this could include a migration path, with backwards compatibility with SMTP, I think it would have a better shot of getting adoption.

Modern email already depends on HTTP, with protocols like MTA-STS (https://www.rfc-editor.org/info/rfc8461/) using HTTPS/TLS to improve transit encryption, or Web Key Directory (https://datatracker.ietf.org/doc/draft-koch-openpgp-webkey-s...) which uses HTTP for public key distribution. Personally I think these incremental improvements of SMTP are more promising than replacements, but we deserve better email however we can build it.