And in 9-12 months, HTML fallback will become optional for senders who want to "optimize" their content. And to prevent "inconsistent user experiences, which are causing user confusion."
Senders can include or not include any MIME types they want; that's just how email works. Though it seems unlikely they'd skip HTML fallback, since not every email client connected to Gmail is necessarily going to support AMP.
I guess that the HTML fallback is not cost free to create. I bet that some of them will be links to the website, like the "read this message on the web" links that are almost the only fallback we have from complex layouts now.
That's what the specification says they should. Just like HTML emails should not have an empty "text/plain" as an alternative to their main "text/html" content, but some mailers do that anyway.
And I do mean empty, not absent, so MUAs don't even try to convert the HTML to plaintext.
Or sometimes, the plaintext version is buggy, eg. because the mailer forgot to remplace {{template variables}}. Or because it sent a boilerplate text. Or the content of a mailing that was sent years ago. (All of these are true stories, I got emails like these.)
The conclusion being that if some format (AMP, HTML, ...) becomes prevalent, developers will stop caring about other formats.
