Does Apple really have to create a "master" key though? Couldn't Apple write the backdoor that would only activates on the iPhone in question? Even if it was so

Does Apple really have to create a "master" key though? Couldn't Apple write the backdoor that would only activates on the iPhone in question? Even if it was something as simple as "if (secure_id == terrorist_phone_id) [accept any pin]", it's not like the FBI could remove the condition without invalidating the signature. If they could, they wouldn't need Apple's help to begin with.