Adapters, like dongles on laptops, are an inconvenience. Bluetooth does not work well in general, provides only low quality audio, and even worse performance on calls trying to stream input and output. A significant amount of people (maybe a majority in the market for non-Android, non-iPhones) still use wired headphones because of these disadvantages to wireless.

Seems like a serious miss with the 4. Hopefully they'll bring the jack back on the 5, unless it's too expensive, hard to waterproof, courageous, or whatever the new excuse is for ditching it.