If .gov LE could articulate and produce real probable cause, that criminal or terrorist information (real evidence), was indeed, present, in the iPhone, business record, safe, safety deposit box, etc, and could get a legally issued search warrant (based on real probable cause) that authorized the search for and seizure of that (real evidence), then the US Constitution says go for it...
And with the legal search warrant issued...
the non defendant holders, keepers, or possessors of such "property" can be compelled to stand by while LE performs the search.
Or can assist in the search, or can facilitate the search, as reasonably prudent,
or can be compelled, in the case of the bank and safe deposit box (specific rare cases), the phone company for call records, or medical establishment for medical records and a few others.
Common sense needs to prevail in the above cases...
An iPhone, sold to a customer, is no longer the property of Apple. The manufacturer of any sold product has no constitutional obligation to LE to deconstruct property for seizure of evidence.
Apple is standing on solid ground and established precedent.
Blunt. It's not their job, to do LE's job.
And LE cant articulate real probable cause to a court of standing there is known criminal evidence in the iphone to get a constitutionally valid search warrant.
Thus the fishing expedition FISA warrants that are extra judicial beyond the Constitution in the name of National Security.
Others here have argued the rest of it. Just when has the intrusive government proven it has the peoples best interests at heart.....