Even if they wanted to, Google couldn't kill HTML in Gmail without breaking a huge number of emails for their users. AMP adoption would have to be near universal before they'd even consider something like that. Gmail, for example, still supports plaintext email just fine even though HTML has been supported in just about every mainstream email client for years now.
Much like “we won’t ever use AMP to rig search results”.
And yes, users are forced to use AMP through their search, for example.