The FBI thought they were asking for a way to unlock a mobile
phone, because the FBI is myopically focussed on past criminal
investigations, not the future of the technology industry, and the
FBI did not understand that they were actually asking for a way to
tracelessly unlock and mess with every ATM and credit card on the
planet circa 2030 (if not via Apple, then via the other phone OSs,
once the festering security fleapit that is Android wakes up and
smells the money).
If the FBI get what they want, then the back door will be
installed and the next-generation payments infrastructure will be
just as prone to fraud as the last-generation card infrastructure,
with its card skimmers and identity theft.
And this is why Tim Cook is willing to go to the mattresses with
the US department of justice over iOS security: if nobody trusts
their iPhone, nobody will be willing to trust the next-generation
Apple Bank, and Apple is going to lose their best option for
securing their cash pile as it climbs towards the stratosphere.