You can generically solve the problem of Alice giving David access to Bob's service on her behalf without giving Alice's credentials for Bob's service to David using stuff like OAuth2, this is already how lots of things work today.

In OAuth2 David only ends up with some token showing Alice authorised David to use this service on her behalf. Bob can tell David and Alice apart, and choose to restrict what David can do appropriately.

If Bob is particularly tired of this nonsense, and his customers like Alice keep giving David their credentials and then are surprised that doing so means Bob can't tell Alice and David apart, WebAuthn reifies it so that most users in Alice's position can now see where the problem is. When David tells Alice he needs her Yubikey to access Bob's service, it should occur to Alice that giving the Yubikey to David isn't a good idea because then she won't have it any more. Good.

I think PSD2 is supposed to solve these problems with a less insane approach, but the rollout seems to be quit sluggish.

Surprisingly, there are already integrations in my home country; I took a look at tink [1] some time ago (no affiliation whatsoever) and they look legit. I'm sure there are more SaaS like them.

[1] https://docs.tink.com/market-capabilities/aggregation

https://plaid.com/ does it well.

Don't they effectively do the exact same thing? As far as I know, they use screenscraping for most US banks rather than something OAuth-based.

I think it depends on the bank. It's really up to the banks to provide a proper API.