You're technically correct, but I don't think it's realistic to separate the payload format (plain unencrypted RFC 2822) from the various transfer protocols that have grown with and around it.
You mention HTTP, but do you think HTTPS have been as ubiquitous as it is now if people had to install a browser plugin in order to enable it? That sounds suspiciously similar to what South Korea has been trying with their infamous ActiveX plugins for their own national PKI. Nowadays they've moved away from ActiveX to actual browser plugins for Chrome/Firefox/etc, but nobody likes it as long as they have to install even a single plugin. "Compatibility with existing clients" really just means "compatibility with existing clients without any plugins". People don't like installing plugins.
Maybe a few years from now, every well-known email client from Outlook to Thunderbird to SquirrelMail will support an E2E encrypted email protocol (either a modified version of SMTP/POP/IMAP or a brand-new protocol) out of the box. Then, and only then, I think it might have a chance to become as successful as STARTTLS has been.
You mention HTTP, but do you think HTTPS have been as ubiquitous as it is now if people had to install a browser plugin in order to enable it? That sounds suspiciously similar to what South Korea has been trying with their infamous ActiveX plugins for their own national PKI. Nowadays they've moved away from ActiveX to actual browser plugins for Chrome/Firefox/etc, but nobody likes it as long as they have to install even a single plugin. "Compatibility with existing clients" really just means "compatibility with existing clients without any plugins". People don't like installing plugins.
Maybe a few years from now, every well-known email client from Outlook to Thunderbird to SquirrelMail will support an E2E encrypted email protocol (either a modified version of SMTP/POP/IMAP or a brand-new protocol) out of the box. Then, and only then, I think it might have a chance to become as successful as STARTTLS has been.