I'm not sure I'd call having a good size screen that every other manufacturer had been offering for a couple of years "phenomenal".
We're discussing sales not design. The sales were phenomenal, the spike was in the sales.
It's probably the headphone jack. People use it a lot, and they see that the iPhone 7 doesn't have one and the "solution" is a chain of dongles or $120 earbuds they have to charge and void losing, and decide to wait and see what happens next year.
Not really. The "solution" is to use the wired earbuds that come with the phone and plug into the lightning adapter. Most iPhone users use the earbuds that come with the phone. For the few that do otherwise, or need to plug into the car, there is the included dongle. There is one big use case where the current solution is a problem, charging while driving and while having the phone connect to the car. But there are 3rd party two way adapters for that.