Probably dumb hypothetical: What happens if Apple comes up with an excuse to remove Uber from the (Chinese) App Store and corresponding iPhones?
The more contentious issue is with builtin apps and platform capabilities, e.g. it's possible Apple's map could show directions and prompt a one-click Didi ride. Google already does something like that with Uber, but it could be even more tightly integrated with payments, and in a way that the platform could favour certain providers.