> Forcing someone's eyes to open isn't impossible but would be difficult

1. Be at border.

2. CBP officer requests to unlock my phone.

3. He/She holds it up to my face to trigger the unlock.

4. I close my eyes to prevent the unlock.

5. The officer presses the end of his/her gun on my head and tells me to open my eyes within the next five seconds.

6. I comply to avoid having my brains splattered on the pavement.

7. ???

8. iPhone is unlocked.

Or, you know, anyone else who wants to unlock it (cybercriminals, pickpockets, etc) can simply threaten your life over a phone (since, let's be real, people have died for less in the first world).

I don't see your point. This is true, but it's true for everything. Passwords, biometrics. Even a physical key.

If your point is no lock is safe from someone saying "I will kill you if you don't unlock this" then yeah, that's true.

Steps 5-8 are the same if you have a passcode lock too.

And step 9 is simple, call the bluff.

I don't see how this is a change from other identifications methods (passcode, fingerprints). If someone is ready to use unlawful violence to get your phone, you have no real way of avoiding it.