However, there is a key that is used to *sign the binaries*. If the FBI subpoenas that, they could sign their own binary that does what they want- including disabling the 10 attempt limit , allowing key entries from other than the touch pad, and removing the delay between attempts.
Once they had that binary built, signed, and installed on the phone they can brute force the encryption - exactly what they've stated they intend to do.
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Arguing that they shouldn't surrender something they have already created is (I believe) a much weaker argument.
That would most likely be fought under as a violation of their 5th amendment rights " nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.". Given the value of the signing key (based on the corporate value of Apple the corporation that would be severely harmed as a result), the government probably doesn't have that kind of cash lying around.
Apple is very much a disinterested 3rd party in all this.
As an information services provider (iCloud) they promptly replied to all of DOJs requests for assistance and information, providing what was available in the iCloud account when it was requested.
As a device manufacturer, their obligation ended when the device left the store. The device and the information on it belong to SB County. SB County can (and has) turned it over to the FBI and said go ahead and get what you can off it. Apple has, in the past, provided services for law enforcement to do that when it was part of what they already did to repair and/or recover phones. They're being told to provide a service that's outside the scope of what they do as a company, even internally for diagnostics (why the pen register argument doesn't apply). Apple don't own the device, and while they license the software that's on it, that license amounts to "you can't copy this". The Federal Gov't is free to use their own vast resources to get information off the phone without interference or help from Apple.
For the government to demand the signing key, when Apple hasn't committed a crime and the signing key is itself not something that was used in the crime, amounts to an act of eminent domain. That's got a whole different set of standards, and the signing key could be determined to be extremely valuable to Apple (as in able to cause a substantial drop in the value of one of the highest valued companies in the world). They'd bring out a whole different set of really good lawyers for that one, and win easily.