In the future, what will prevent some random border officer from pointing my phone to my face and unlocking all my social media, google/Apple accounts, mail, bank etc?

What's stopping them from doing that now?

My password, pin, whatever, is in my head

In that case you should probably not use this, or if you do, sign out of your accounts. If you're worried about Face ID, you can actually temporarily disable it easily (without even looking at your device) by holding down the power button and volume up until it vibrates.