>> There's legitimately no alternative, "secure" way to access someone's banking data other than by asking for a username/password

Not so true under Open Banking and other systems. Open Banking uses OAUTH2 style stuff, and a variety of certificate schemes (Open Banking's own, and EU ETSI qualified certificates) to allow participants to be delegated various forms of access and roles, in a much more granular way.

>> As a commenter on the issue says, there is no oauth-esque mechanism implemented by banks.

There is in the UK, and PSD2 solutions are rolling out all across the EU. Australia is also getting in on the action.