Okta already supports what you suggest. The problem, IMO, is the lack of standardization. Okta has its own implementation, Google has its own implementation,

Okta already supports what you suggest. The problem, IMO, is the lack of standardization. Okta has its own implementation, Google has its own implementation, etc. TOTPs in stark contrast are pretty universal and not that much harder to deal with, provided you have some way to backup and recover your TOTP keys (this is Authy's selling point, for example).