> The biggest vector MFA protects against isn't really brute forcing (though it helps there); its password phishing. This is literally the only reason behind why Google's "account compromises dropped to zero after we required MFA internally" thing is a thing.
To be clear, there are phishing resistant authentication methods. Specifically:
1. Kerberos
2. Mutual TLS
3. Web Authentication
All other authentication methods (passwords, SMS, TOTP, QR code scan) are vulnerable to phishing.
Google's announcement about dropping to zero wasn't about adding just any new MFA (they already had MFA) but in mandating Web Authentication.
Some people get confused about phishing risk because passwords are vulnerable to passive phishing (e.g. log to file to attempt later), while the other mechanisms on this require active phishing (connect to a service in an attacker's browser, and parrot that authentication flow to the legitimate user). These active attacks are becoming more common, as you would expect.
Password managers greatly reduce the susceptibility to phishing because they care about the site that one is currently on; the person will have to perform different steps to release their password (or TOTP) to the wrong site. There is a proposal to put semantic tagging in SMS challenges such that platforms will only offer to auto-fill on the specified site for the same reason.
However, the three authentication methods above do not have user overrides such as manual SMS code entry - they simply won't work. You instead need to instead compromise the service, fronting application or hosting infrastructure/certificates.
That isn't to say that the phishing-susceptible secondary factors doesn't help the site in other ways. There is simply no good way to guarantee that someone isn't reusing their email and password on several other sites until the password shows up on a breach list. This is especially a problem for corporate logins (using corporate email addresses).
However, such secondary factors don't provide any benefit to the user with good password hygiene.