Starting with the iPhone 7 and, we presume moving on to newer generations, there will no longer be a headphone jack on iPhones.
There are lots of reasons why Apple removed the headphone jack. Presumably, the major reason is that the jack is taking up too much space on the inside of the phone. And because the jack is round and relatively fat, the phone must be at least 5.5 mm thick in order to have a jack.
Every generation, the iPhone gets thinner and thinner. Pretty soon, the headphone jack will present a design limitation for Apple engineers. Of course, there are other reasons they removed the headphone jack. Some are more acceptable than others.
Aside from saving design space and allowing for future phones to be slimmer, there are several other good reasons Apple had to remove the headphone jack.
First, the phone can be made to be more rugged without the headphone jack. Imagine being able to go scuba diving with your phone and taking underwater pictures. Or what if your phone could survive almost any drop onto pavement? By removing the jack, Apple can make the phone tougher.
Apple also wants you to start getting used to wireless headphones. Wireless headphones are way more freeing than wired ones. Plus, wireless headphones allow your headphones to integrate well with other technologies (like the Apple Watch and other future devices).
For those with premium wired headphones, you will now have to use a dongle to listen to your music. Not only is this inconvenient, it can be an outright hassle. You could lose your dongle or, at least, not have it on you when you need it. Then there's the old home stereos. Lots of people have headphone adapters that go into their old stereos. Now that your phone doesn't have a headphone jack, you are out of luck unless you have a lightning-to-headphone jack converter.
Even if you are using an adapter, you can't charge your phone and listen to headphones through a jack at the same time. Some adapters will allow you to do this, but they can be expensive or low quality. Watch out for low quality charger and headphone jack adapters. The headphone jack may have a built-in buzz while the phone is charging.
Overall, some functions were gained and some functions were lost. There are many specific options and choices that you have to handle this inconvenience.