Why doesn't the FBI simply clone the current device, make brute force attempts and then clone again if locked out? Yes, lots of work but also doesn't force Apple to participate.
http://forums.appleinsider.com/discussion/comment/2832533/#C...
“Each Secure Enclave is provisioned during fabrication with its own UID (Unique ID) that is not accessible to other parts of the system and is not known to Apple. When the device starts up, an ephemeral key is created, entangled with its UID, and used to encrypt the Secure Enclave’s portion of the device’s memory space. Additionally, data that is saved to the file system by the Secure Enclave is encrypted with a key entangled with the UID and an anti-replay counter.”
https://www.apple.com/business/docs/iOS_Security_Guide.pdf
The device in question is an iPhone 5C, which uses the older A6 design.
Everything is encrypted with a derivative of this UID, and extracting the UID is not a thing you can do without destroying the device.