Email is not controlled by Google. There is myriad of other big players in the field, plus you can get an email server on your own domain up and running in ~ 5 minutes

Email is controlled through gmail in the same way as webdesign is controlled by SEO directed towards google search. People need to access the users on the google platform, and google sets the terms for that.

That is, if you want your email to work you got to play by the rules dictated by google or a very large portion of all email will go away into a black hole.

If Google will start blocking random non-spammy domains from exchanging mail with Gmail users, it not end well for Google and their mail platform.

I imagine that Google is waiting with bated breath for the day when their user base is large enough that they could get away with this, and are sitting, all day every day, with their finger hovering over the button.

It is our role to make sure that they never get sure enough of themselves that they press that button.