FYI - Sending mail via G Suite - Google announce username/password authentication will be turned off
CiviCRM is not responsible for authentication of users (that's the CMS, ie. Drupal or WordPress). Using OAuth2 (or other protocol) for authenticating CiviCRM to another system would be the responsibility of the extension that wants to do that.

Or do you see a reason to include Oauth2 in core CiviCRM?

I'd be hesitant about using Oauth2 before fully understanding what you're getting into - here's a helpful blog w/ links from Colan Schwartz:

https://colan.consulting/blog/authenticating-drupal-users-oauth2