Oops, I meant to say deccelerated. But it appears to be worse - stagnant at 14%. So, I think the real reason they need to ask themselves - why? They've tried a few things with low cost iPhone SE models to gain market share but it has failed thus far. Perhaps it is the inertia of switching OS/GUI, or perhaps it is an indictment of how bad the iPhone UI has gotten. Or, that Android and other phone manufacturers are offering mutually exclusive things that iPhone simply cannot satisfy. It surely isn't the shortages.
Or perhaps we need to be asking ourselves: Why do we demand ever-increasing growth in order to declare that a company—hell, even a product or a segment within a company—is "healthy"?
Smartphones hit a saturation point a while back. At this point the only ways for Apple to grow marketshare are to a) lure away current Android customers, or b) convince first-time smartphone purchasers to go with iPhones at a higher rate.
Neither of those things are trivial, and not being able to do so for a little while should not be taken as an indication that something is wrong.