What you describe is already how the iPhone works. The FBI has vaguely threatened to seek the signing key (not any master encryption key, which doesn't exist) so they can make an operating system that disables software-imposed wipe-on-10-incorrect-attempts and software-imposed-delay-ramp-up, so they can run a brute-force attack. If the phone had used a long passcode, this would be impossible. The only reason this is even a case is because this phone has a numeric passcode, which is within the reach of brute forcing with these UI barriers removed.

The mathematics already impose an 80ms duration ("delay") of each attempt. The FBI knows there is no way around that.

Contrary to other replies, even phones without the secure enclave are protected in this way.