Lots of people have lots of opinions on what is and is not “constitutional,” most egregiously post-Snowden. But unless you’re a Supreme Court justice, those are really just opinions, aren’t they? Plenty of people would disagree with those “scholars” (many of whom are also activists) who are most vocal about the illegality of NSA surveillance.
Cook’s “master key” framing of the issue is also probably deliberate. No one likes the idea of some faceless bureaucrat holding a magic key that can unlock any phone, anywhere. There’s no reason why a potential system needs to be structured like that, though. Today, when law enforcement needs a wiretap, they just go to Verizon/AT&T/etc. with a signed, valid warrant from a judge, and those companies comply with standard assistance. There.