There's not much you can do with the IP if China stops being willing to make the product for you. (Unless the IP is so far ahead of the competition you can sell it, but Chinese phone manufacturers are making phones that are very competitive with iPhones.)

Even the Chinese phone manufacturers rely on American software (Android), even if Google gives that away.

If China refused to assemble iPhones for Apple, that assembly could easily be shifted somewhere else: the components are mostly not from China and even the machinery used to assemble the iPhones is mostly German. Apple/Foxconn do it in China because it is the best deal ATM, and shift iPhone assembly around as needed easily (eg iPhones are assembled in Brazil, and will be in India to get around tariffs).