The 3.5mm headphone jack is said to be a universal jack, where you can find it just about everywhere, but eventually, people need to move on, right? Perhaps Apple feels that there will be not much of a future left for 3.5mm headphones in the future, assuming this rumor is true, but if that happens, then a whole slew of audio accessories would be rendered useless on the iPhone 7 in one fell stroke.
Alternatively, doing away with the 3.5mm headphone jack would mean it is an effort to slim down the iPhone 7 further, which might eventually see the next generation iPhone end up being one millimeter thinner compared to the current flagship, that is the iPhone 6s.