I don't see the problem.
If the FBI, or anyone else, wants to take my phone away from me and invest a lot of time and effort into physically dismantling it and brute-forcing any encrypted content out of its memory, they can do that. They've got the resources. I don't mind if phones are not built to resist that level of attack.
If they don't want to go to those lengths, then obviously their motivation for wanting to know isn't strong enough, and they can stay the fuck out.
Problem solved, no?