Google isn't the only one implementing AMP for email, nor are they the sole controller of what AMP implementations look like. It's an open spec. Yahoo, Outlook, and Mail.ru support it.
Unfortunately "HTML + JS" based is no longer open.
Commercially competitive web browsers are absent on open (not Apple) platforms which leaves the Chrome + Firefox duopoly. Somehow Google will help Firefox stumble along since it will keep them out of trouble with the antitrust courts.
