Earlier quoted context omitted.
It seems obvious to me why they don't use that for everyone, and it has nothing to do with security.
I don't really agree (and I'm happy to bash on Google). This is basically the poster child for a case when someone should be using 3rd party cookies: A single entity manages multiple domains and shares cookie auth across them. It's not like the other flow is somehow making you less identifiable - they're literally just passing the same information in a more round-about, less usable manner. I genuinely think the curre…
If everyone would use 3rd party cookies like you're describing, there'd be no issue with users enabling them. Instead, they're frequently used to track users across domains, and the alternate flow used for Safari should be the pragmatic option used for everyone.
You're right to complain about how we're basically unable to use an otherwise-useful feature because of bad actors. It's a signal that core web technologies need to be created with potential abuses first and foremost.