Exactly. There's no engineering reason, and if there was, it would be embarrassing. Apple is a marketing and advertising company at this point which makes hardware and software to support that mission. So they marketed that they wanted or needed to remove the jack because <reasons>, when the real reason was that they wanted to sell you overpriced bluetooth headphones and leave no option otherwise.
They added a more sensitive camera that year as well as a brighter display with more colors, found interference, moved the display circuitry and cabling to the other side of the phone to compensate, and then that interfered with the headphone jack instead. So they took the headphone jack out instead of compromising the quality of the audio output.
If the guy who put the jack back in did any actual signal quality testing while playing video to stress the display, I'd be appreciative to have this theory put to rest, but I don't think he did.
