Fair point, but I think the AirPods case will get some screen eventually, even if just to display battery life. Screen technology is becoming so cheap that at some point, why not include a display to provide basic information, and from there, features can grow year-after-year.
Alternatively, imagine that the phone becomes the case. Take the original iPod, for example. It was large enough in size that today's AirPods could have fit inside that iPod for charging/storage. If AirPods continue to get smaller for the ear, perhaps one day they'd fit inside of our iPhones.