Settings and pop ups mean that users have to understand them - which can be a huge user education challenge. I'm pretty skeptical of the argument that it's simple to add in a new user facing pop up and have the majority of users use it correctly.
Nevertheless: even if a solution turns out to be too difficult for users to understand no matter how you present it (at which point you might want to rethink whether this apparently fundamentally flawed idea should even be implemented at all), violating your users' agency by taking the reigns and obfuscating the whole thing to them is morally wrong and in simple terms: an arsehole move.
I'd see a browser side system that could actually more user friendly and more unified than the current SSO situation.