Do you think the NSA hasn't considered that possibility? The US intelligence community has been seriously pursuing code breaking since the late 1930's. Their interest and resource commitment has only grown. Bletchley Park had a half dozen or so hand built 'bombs' working on Enigma. The US was rolling them off production lines hooking them up to IBM card readers and sorters and working at industrial scale.
Encryption software and hardware has to come from somewhere. Maybe it's not the US. What then - China, Russia, Great Britain, or perhaps Israel? Who else has military grade capability? And who won't backdoor it?
Recently I've been reading history. The Sioux warrior Red Cloud shut down the Bozeman trail. Then the weight of 10 million Americans overwhelmed his nation of 20,000. It's hard to grasp vast differences in scale. The NSA isn't Apple or Google.
It can raise billions to combat bogeymen and need not turn a profit let alone answer to stockholders every quarter. It can appeal to a patriotism that runs far deeper than mere brand loyalty. Nobody is going to put their life on the line for iOS.