I don't think it's a failure of design, but rather a symptom of too much functionality.

The original iPhone had ~30 features it had to make work in harmony with each other. The current version of iOS has thousands. Each additional feature is an increase in difficulty in making it feel harmonious, and it's beyond the remit of what a single designer with a single vision can handle. It's grown to a scope such that no single person can contain a mental map of every aspect of it at once.