The FBI has the hardware. At the software level it should be game-over. So what is stopping them from copying the phone's memory, putting it in an emulator or another phone, and brute forcing the 5-digit PIN. Every time it self destructs, they load up another copy and continue until the correct PIN is found. What am I missing here?
Apple has stated that anything with an A7 or newer CPU has a unique code embedded in the hardware that is combined with the PIN to serve as the encryption key. Apple doesn't record the hardware key, and they are the only ones that possess the keys for the software used by the secured enclave in which it resides. So without Apple's help, the DOJ would have to first break into the secure enclave, which I presume is so difficult as to be impractical, and only then could they try the brute-force method you described, which would be much easier.