The FBI isn't quite telling Apple how to build software in general. They're trying to make Apple build a specific piece of software for the FBI.
If they do this today, they can probably do it tomorrow, yes. But "this" doesn't mean compromising all iPhone hardware or software. It means building a special copy of the software to attack already insecure hardware and software.
Let's say Apple makes the iPhone 7 completely invulnerable to attack even from Apple. How does this precedent play out? The FBI goes to Apple and says, "Do that thing you did before." Apple replies, "Sorry, but we literally can't do it for this phone." How does the existing precedent cause a problem?
This whole line of argument feels too much like, "The FBI can demand something, this is something, therefore the FBI can demand this." People keep saying there's a connection and I just don't see it. When I ask what it is, people just reiterate that it's there.