If the new phone doesn't have a headphone jack, it'll be all over the Internet. There will be almost no way to avoid knowing that the iPhone 7 doesn't have a headphone jack.
That's not where the user impact comes in. Most people don't use headphones constantly. They use them occasionally. And they will think to themselves, "That's not a big deal." Then, at some point in the distant future:
- They're at a friend's house and want to play some song. Their friend has an Android phone, and a stereo with only an 1/8" plug.
- They're out somewhere and think, "I'd like to listen to some music while I walk from A to B" and then realize that their Bluetooth earbuds aren't charged.
- The stewardess tells them that they can't use wireless headsets (that's a per-airline policy decision) and offers to sell them a headset for $3, but oops, no adapter.
And so on. And suddenly, what seemed like it didn't matter suddenly matters, and you have a pissed off customer.