Who says it's only about phones?

The fact that it's an iPhone in the San Bernardino case is kind of a red herring; the precedent it would set is not "iPhones and only iPhones and not any other type of computing device are subject to law enforcement unlocking". The precedent it would set is that any encrypted system must be built to be decrypted on demand by law enforcement.

And that gets back to my original comment: a system the "good guys" can decrypt is one the "bad guys" will be able to decrypt too.

Slippery slope arguments are lame when they start to go like this.

Like, I get you. By allowing X which might actually be OK, we might allow X+1 to happen and that wouldn't be OK. So we must oppose X as a practical matter.

I get the practicality of that.

But when you start opposing X as a matter of principle rather than simply as a matter of practicality that's when you've gone off the rails.

Well, for one thing it's not even a slippery slope. They want to unlock his work-issued phone, not his personal phone, and the FBI admits they have, and other law-enforcement agencies admit they have, an almighty queue of other things they'd like to get force-unlocked if they can only get precedent claiming that companies do indeed have to circumvent their own products on demand.