It's taken us something like 30 years to start seeing wide IPv6 adoption. Even then most a running dual stacks to still support IPv4. How long do you think it would take to have a secure email protocol running by most users?

It could probably happen quickly if a few big players came together and made the push. Unfortunately the only innovation in email these days is occurring in ways that help user lock-in. E.g. IMAP is stagnant, Gmail added 'dynamic emails' that only work in their ecosystem.