Google will not be able to kill smtp, gmail is only one side of the email market. They’d have to offer a transactional and marketing email service and get every app/company to use that over smtp. It’s monumental and I don’t think it will change for decades.
You can damage open/federated email without having to break the lower level layers. See winmail.dat for instance[1].
I don't know if it's still in the pipeline but I remember that Google proposed
an "AMP for email" not so long ago, with basically self-updating email contents.
Open email can be severely damaged if good old SMTP is only used to tunnel
proprietary formats, especially if those formats are effectively just metadata
that's used to fetch the actual content of the email from 3rd parties.
