majority of people have switched to wireless audio
Citation needed. Also, just because people have “switched” to wireless doesn’t mean that they don’t have a pair of old wired headphones still lying around somewhere. Also, I suspect a significant portion of Fairphone users are the kind who’d still hold on to wired headphones.
when you’re trying to make a planet-conscious product?
The first rule in making a planet-conscious product is the RRR - Reduce, Reuse, Recycle. If people already have wired headphones, then the most eco-friendly solution would be to enable users to continue to use them, and not force them to buy new products. And as a manufacturer, there’s practically no shortage of 3.5mm jacks around (plenty of old devices where the parts can be recycled from), and there’s almost no complexity involved in wiring up or making circiuriry for something that’s been a standard for several decades.