Nobody's "forced" to use it. Users can turn off AMP in Gmail, and email authors can continue to just use HTML emails if they don't like AMP. HTML emails will continue to display in Gmail just fine for the foreseeable future.
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.
