Counter Point: Until people can migrate their inboxes and steer them to any provider, none of this authentication business seems to hold up actual value at scale.

If anyone can port their phone number, they should be in theory, allowed to port their email addresses as well.

None of the authentication systems here are helpful enough to allow this. You need a valid way to authenticate people irrespective of whatever provider they are on (not their email domain name)

That means that a standard needs to evolve that allows you sign on the behalf of the hosting provider itself.