Apple has an excellent mobile OS, which is enough of a loyal userbase that they can make a hardware mistake once in a while and still retain customers. They're less hardware-dependent than most device manufacturers. This also enables them to lag behind the state of the art if it means more reliable/consistent performance. Which is why you don't see a folding Apple phone yet, and why Samsung was able to score points against Apple by having longer battery life and a better camera. This also allows them to demand high quality from their factories.