The legal question here is really whether the All Writs Act can be used to compel development of new products with specific features law enforcement desires, because that's what it boils down to: they want a custom iOS build with custom security-bypass features that can be loaded onto any phone they want to snoop into.
And when framed like that -- government ordering a private company, without negotiation or contract or even agreed-upon -in-advance compensation, to develop a product -- I don't think there's a chance in hell of it standing up. The question is whether the court that ends up hearing the final appeal will frame it like that.
How is it different than getting a phone company to install a wire tap?
What you describe is not what's being asked for. What the FBI wants is a tool to get into this phone. They will definitely want to get into more phones in the future, but that's not the same as a single tool that gets into any phone. It can be a new one every time.
If it wasn't possible to make such a tool, I'd be a lot more outraged. But it is. Tell me why that can't be done.
The difference is simply that telecommunications are heavily regulated (see CALEA) where if you want to be a service provider you have to provide wiretap functionality.
Personal computers in general are not covered by any such regulation. It is legal to create and use encryption on your own device without building in a government backdoor. For now.
Ironically, the biggest threat to an unregulated computer future is the massive centralization of device control via platform DRM, in the hands of just a few companies like Apple.
CALEA was extended to ISPs once the industry coalesced into a much smaller number of central players.
The difference is that tools and equipment for wiretaps already exist. Tools that bypass the security features of more recent versions of iOS don't. Hence the FBI's desired insecureOS would be a new product, developed under compulsion from the FBI and the courts.
And when framed like that -- government ordering a private company, without negotiation or contract or even agreed-upon -in-advance compensation, to develop a product -- I don't think there's a chance in hell of it standing up. The question is whether the court that ends up hearing the final appeal will frame it like that.