> how do I log in on a device that I don't own?

This is solved by passkey-implementing software and devices (with Bluetooth) allowing you to log in with a QR code (Webauthn via CTAP hybrid transport). iOS and Android support this, and it’s generally not a locked-down thing if other devices wanted to do it too.

The only use case left is in “how do I login if all my devices are stolen/fall into a body of water” in which there really isn’t an answer beyond “get (a|your) device back, sign back into your password manager, use that to get back into critical accounts”.