Yeah, they won't go away, but they probably won't change to use a USB C connector. Instead they will stay at 3.5 or 6.3mm. That means you need an adapter anyway

Yeah, they won't go away, but they probably won't change to use a USB C connector. Instead they will stay at 3.5 or 6.3mm. That means you need an adapter anyway to connect those headphones to the device. Now there's the valid question why you want to produce a passive adapter that is only working with devices that support analog audio on USB C instead of producing an adapter with a DAC that works on all USB C devices. I guess the answer is as usual: Price. The manufacturers might be flooding ebay and co with supercheap adapters. And the end users will as usual have trouble to find out which ones will work with their phones and which not.