But I think that the government should be able to require Apple to use its capabilities to hack particular devices with a court order.
That's not the question, though: the question is whether the All Writs Act already grants that power, or whether a new piece of legislation is required.
I think that Apple is right that it is more conservative for the courts to construe the existing legislation narrowly, since the Government always has the option to put specific legislation before Congress to remove all doubt.
Legislation shouldn't be construed broadly or narrowly. It should mean what it says. Sometimes that's called 'narrowly', but that's probably in contrast to approaches where the law means whatever you want it to mean. The All Writs Act is a broad law. It grants the courts the broad authority to issue whatever orders are "necessary and appropriate" in order to enforce their judgments, so long as its "agreeable to the usages and principles of law." Congress is free to pass revisions to the All Writs Act if it pleases.
It's all very well to say "It should mean what it says.", but this is just begging the question - frequently what it says is imprecise, so the meaning is open to interpretation. In the case of the All Writs Act, for example, the Supreme Court found a three-pronged test in New York Telephone.
It's precisely because it is a very broad law that it should be construed narrowly - in this case, that it is intended to be able to compel the co-operation of parties with some real involvement in a case, not to authorise open-ended civil conscription in service of the judicial branch.
That's not the question, though: the question is whether the All Writs Act already grants that power, or whether a new piece of legislation is required.
I think that Apple is right that it is more conservative for the courts to construe the existing legislation narrowly, since the Government always has the option to put specific legislation before Congress to remove all doubt.