That's an interesting point. As more and more wired earphones switch to USB-C, there shortly uld be a way to adapt them back to 3.5 mm if needed. I'm sure there is something on Amazon or eBay, but I'd like to see them from OEM manufacturers too.
I think that you are suggesting the ability to plug your USB C headphones into a 3.5 mm jack on the plane.
The problem I see is that USB C provides power. Plugging into a 3.5 mm jack wouldn't supply the power avaialable at a USB C port.
The other thing is that most air carriers are switching to a bring your own device paradigm. You watch their content over Wifi on your device. I think the need for a passenger to connect directly to an airplane's audio system is fading quickly.