All the "Push notice" 2FA systems I've used or looked at are already cheerfully bypassed by systems for phishing TOTP and SMS 2FA.

That's because they're relying on a human to make a security decision and humans are terrible at this job.

The human is at fake-site.example which they think is the real site because humans are lazy and incompetent. The fake site's backend automatically calls the real site, and the real site sends the human a push notice as 2FA. "Are you signing into Real Site®?". "Actually I wasn't sure until I got this reassuring message" thinks the human, "Yes". And now the bad guys have access to their account.

There are a lot of HN stories which are dubious about AI because they suspect it can't ever match a human in terms of vague things like intuition and learning. But you know what computers are unmistakably good at? Trivial symbol matching. fake-site.example is not real-site.example, those strings are different. That's why you just can't use your real-site.example WebAuthn credentials on fake-site.example