The big problem is that Apple would have to move or rebuy a huge wallop of high-precision CNC machines that they have installed in China.

http://www.iphonehacks.com/2016/10/next-iphone-probably-wont...

Koenig writes. “Apple is such a huge buyer of a particular kind of mill (BT30 spindle drill-tap centers) that Fanuc, Brother and DMG Mori each have factories dedicated to building machines exclusively for Apple.”