Ethernet makes less sense on a mobile device, because you don’t carry routers on your body, and the standard was no where near as old. Again, removing because there was a better alternative. Adaptors are not a better alternative. Bluetooth is not a better alternative.
Your metaphor is dropping important details. The cable mattered because it tethered you to a location, which is not true of headphones. Routers also didn’t need to be charged before or after.
The headphone cables tether you to your phone. There are times when you want to be able to put down your phone and be able to use headphones but not be physically connected to it. Eg lying in bed listening to music, but then getting up to go to get something from the kitchen (and not having to pick up your phone, or carry it with you, or unplug the headphones) before lying back down again. And being able to do that while continuing to listen to the music.
Routers need to be plugged into a wall socket. Batteries don't require that constant use of a cable or socket.
