The Fairphone (Gen. 6)

If they'd just provide a physical switch (not software-based but actually cutting off the respective chips and antennae from electricity) to go full offline (no GPS, no Wifi, no mobile connection, ...) they'd effortlessly at least double their market potential.

I've never heard this request from anyone before, so I guess that implementing such a switch wouldn't "effortlessly at least double their market potential".

What a lot of people talk about is a headphone jack. But even that niche has been filled by USB-C adapters for people that really want them and not only talk nostalgic about it.

It hasn't been filled by those adapters. Usb-C adapters suck as you have to carry them with you, they can be lost, quality is often bad and they block the one charging port of the device.

The demand for a headphone jack is fueled by functionality and sustainability concerns, not nostalgia - can't, too recent a change and current devices do have the port.

Plenty of phones with headphone jacks had terrible DACs. If you care about audio quality, external is the way to go. It’s also more sustainable as you can spend $100 once for fantastic quality and keep it forever rather than insisting that each phone have an audiophile quality DAC that will eventually be disposed of.

The one upside is convenience of not having a separate dongle, which is pretty well offset by the significant increase in phone size needed to accommodate the jack.

Given the current phone sizes compared to the phones from before, that all had the headphone jack, the idea that you would need even bigger phones as otherwise it just would not fit is ridiculous.

For the external DAC, you have to balance the "you could buy that once" against all the consumers that are pushed by the omission of the headphone jack to buy throwaway head- and earphones with glued in batteries. There is no chance in hell that the waste produced of both paths is in favour of the jackless phones.