This already exists and is called FaceID and doesn't have the same privacy risks since the face check is done locally to allow the user to use the passkey to authenticate.

>Your selfie video is encrypted at rest

All data nowadays is encrypted at rest. That doesn't really matter since someone stealing a hard drive from Google with your information on it will never happen. The more likely risk is Google decrypts it and then sends it somewhere it shouldn't go and potentially trained into some AI system.