Ah yes, and Shibboleth is another I should've thought of in earlier comment.

I think you're absolutely right in particular with #2.

But if it had come originally, alongside DNS, 'everything has an address, everyone has an identity', it might've been an unquestionable fact of the internet.

Orgs can't silo you in to their alternate net where they have a more desirable domain name, because it's just not practical or user friendly.

I just think it might have been so for user identity.