You are missing something, but in this kind of listing it is easy to do that. Apple is including a Lightning to 3.5mm Analog jack adaptor. Your old connections will work as expected when plugged in to that.
Too bad you can’t charge whilst listening to those headphones! And, per the Apple Accessory Interface Spec (R25, the current one) you CANNOT do “Y” cables. Each cable must have one, and only one, connector on each end.
I guess there could be a big dongle with several cables going to it, a little Lightning hub to carry around. Plug your phone into the hub, plug a charger into the hub, and plug your headphone adapter into the hub. So I guess that would work - just at the cost of two donglish things and two additional cables to carry around...