Hardware in general seems to become more commoditized. In China Apple has lost market share to local firms that produce cheap phones with good specs. And most people there spends a lot their time in WeChat anyway so the phone's OS isn't as important.

As this commoditization happens over time, hardware also seems to reach a point of just being good enough. You can see this today with people deciding that their iPhone 6 is good enough and they don't need an iPhone X.

Apple obviously realizes this and is trying to diversify its revenue with increased growth in its services area, but a lot of those services are directly tied to its iPhone. If people buy fewer new iPhones that is bad for Apple, and if people buy a cheaper competitors' phone that is doubly bad for Apple because it doesn't get services revenue from that person either.