Well, the advantage is that since e-mail is a plain text medium, it could be "forced" if clients started supporting it.
And it would require zero hacks like this AMP thing and would also be 100% retro compatible if a markup language with sensible plain text legibility were chosen.
Up to your ability to modify the client there's absolutely nothing that prevents you from sending a multipart email with text/markdown as one of the options. Shoudln't break any existing email clients.
Obviously very few clients will be able to read it, or even acknowledge that it exists but it's something. Maybe someone on the other end will appreciate it.
And it would require zero hacks like this AMP thing and would also be 100% retro compatible if a markup language with sensible plain text legibility were chosen.