The difference is that the iPhone 7 is not targeted to teens and pre-teens. Note: I'm not saying that those are the only people who buy them, but the iPod Touch has become the gameboy for many youth. THAT demographic uses headphones/earbuds frequently and will not look kindly to having an adapter to buy and/or lose.
That is true, there are already Lightning ones included, so you most likely have a backup.Most kids those age just have cheaper iPhones instead of iPod touches. Again, this is where the iPhone SE comes in. And if they keep the body style the same, then the headphone jack will remain and there will be no need for that adapter. But even so, they solve the needing an adapter problem by providing the Lightning EarPods with the phone.
I just don't think it's that big of a deal to most. They can keep the adapter connected to the headphones at all times if need be. Losing headphones was always a possibility, and it might be a bit of an annoyance, but I think even younger children can manage.