Of course if you’re one of the platforms, owning some or all of other people’s login processes does happen to be a win for you.
Ideally there will be third-party credential databases (just like with password managers), and a way to export your keystore and import it into another provider. That would solve this problem.
Technically you don't need a third or even a first party for that; you could self-host. In practice though, the overwhelming majority of users will be using some sort of cloud storage service. A third party service in this case is likely better than first party since I don't want to lose my credentials if I decide to use a different OS or browser, and browser/OS vendors haven't been great on cross-platform compatibility for credential storage thus far.