That is pretty naive assessment Mike. Have you ever lost your wallet somewhere, only to retrace your steps and find it later in perfectly good order, but all the cash missing? There are lots of dishonest people out there that will take advantage of an easy opportunity if the situation presents itself. That is just a cold hard reality - as is finding yourself in a compromised situation, beyond your control, and your phone potentially being found and unlocked by someone else.
Of course the same could be said of fingerprints, but if Google's face scan is as easy as it sounds, the phone would only have to be close to you and not even directly facing you to unlock. So quite a bit less intrusive (and a lot faster to go unnoticed by others) than fumbling with someone's unconcious or sleeping hand and trying to test which finger unlocks the phone. Someone could easily unlock your phone, remove it from your Google account, perform a factory reset, put it in their pocket and walk away - and you would be none the wiser. And that is just the tip of the iceberg.
Again this is a huge ball dropped by Google if they have not accounted for such a scenario (they certainly aren't talking about it if they did). More importantly, there is no way a phone at that price justifies being a guinea pig for for new facial scan tech that probably needs several major updates before it's ready to be trusted, IMHO.