I don't see the slippery slope here. The government is asking Apple to do something that is both possible and reasonable. I see no slope to that from other typical court orders.
Giving the government a way to brute force PINs wouldn't break the trust of every iPhone owner, merely the owners of iPhones with pre-A7 CPUs. And great, if they trusted Apple on this their trust was misplaced. You can't trust companies not to unlock stuff when the government requests it with a legitimate court order. If you want Apple not to decrypt your data, the only way to ensure that is to make it so they can't.
Again, Apple has (so far as we know) made it so they can't, on newer hardware. But this phone that the FBI is trying to get into is older hardware and built such that Apple can get into it. If you're looking to point fingers, blame Apple for building not terribly secure hardware. But don't point fingers too hard, because they're doing it a lot better now.