The attack is simple: the attacker downgrades the phone to a version of firmware that has a vulnerability. The attacker then uses the vulnerability to get at your data. Your data is PIN-protected? The attacker uses the vulnerability to disable the PIN lockout and tries all of them.

There's over a 10x difference in fence price between a locked and unlocked phone. That's a significant incentive/deterrent.

Don't pixels have a security chip that is supposed to make that infeasible?

It has some increasing timer for auth, and if you try and factory reset it - it destroys all the data?

As I said its less important that the thief can boot a new os, the security of my data is more important. How is that compromised?

It feels like a thief is just going to opportunistically grab a phone from you rather than analyse what device it is.