The big objection I can see to the FBI's demand that Apple create a version of iOS without password-guessing protections is that the government is "compelling speech". While I'm not sure of the U.S. legal/constitutional principles behind it, one of the ways that warrant canaries work is that the government cannot compel speech, ie. the government cannot force you to say something against your will, such as a librarian making a false statement that the FBI has not requested information about borrowers. Since computer code is covered under copyright law it has the protections of speech, and so the government should not be able to compel speech in the form of compelling Apple to create specific computer code. As someone said, let the FBI do the coding itself!