Right now isn't it a cat and mouse game of holding many domains/subdomains, and passing the cookie flag across them. It seems technically possible to cancel out the protections of ITP. However, deciding cookies can never be used outside of 3rd party context may mean I have to make some additional logins to services from time to time, but have much better tracking protection.
Is my assessment faulty?
It's been the long-standing default of Safari to block 3rd-party cookies. As is described in the excellent post this links to, there is some functionality that 3rd-party cookies enable that can be beneficial. They use single-sign-on as an example.
But social network widgets (Like buttons, comment form) would break without third-party cookies.
OAuth is typically a user-initiated action, like "Sign in with GitHub" or "Sign in with your domain" (https://indieweb.org/IndieAuthProtocol).
Though I think SSO should be possible with OAuth — maybe with a hidden iframe that does the auth process, or something with CORS requests… Or maybe a custom redirect-based protocol would be better.
Some of the things that would break without third-party cookies are social network widgets - you would be unable to like some post or add a comment using Facebook login on a third-party site.