Bu you would have 1:1000000 chances to get into someone elses phone. With 7 billion people in the world it's not so crazy.
The Secure Element would need to be upgraded to support "multi-tenancy".
That's all assuming the feature works by building on the existing infrastructure.
Ideally between your face and your iCloud password you could bring your "profile" to new hardware without having to touch an existing device. After the first time, just your face is enough.