1. Uber would file a lawsuit for a huge amount of damages. (They have enough money to be able to afford lots of lawyers.)
2. A large percentage of Apple's customers who are also Uber customers would be hugely pissed off and switch to Android phones.
3. There would be huge amounts of negative publicity. There would be congressional inquiries into anti-competitive behavior, and demands for government regulation. Apple's board might even fire the CEO.
It's almost inconceivable that Apple would do something so harmful to its own self-interest.