Analog-only headphones (mid-range to high-end ones at least) will never go away. People who care about having good headphones also care about having an equally good DAC and amplifier to go with them, and they're not going to be willing to pay the premium for decent ones every time they buy a new pair of headphones (good DACs and amplifiers are NOT cheap), nor will they be willing to pay for a mediocre DAC that they never use.
It would be suicide for headphone companies to stop producing analog-only headphones, unless they can force everyone to scrap their analog-only sources (it might be possible with consumer toys like smartphones, and they might be able to convince gullible audiophiles if they can find the right marketing gimmick, but it will never happen in the professional world).
And what about all of the analog-only headphones already in use? Quality headphones are not disposable products (they can last for decades when properly maintained), and nobody in their right mind is going to throw them out until every last analog output in the world has been magicked out of existence.
Anyway it doesn't matter. If you want the best quality, the source (i.e. the phone) has to feed a digital signal.
Then it's turned into sound by either a $10 earphone or a $1000 Hi-Fi system. How well that external device amplifies and generates the sound is up to that external device, they both get the same (good) quality source.
And if your sound system doesn't support digital, you can get a USB-to-Jack35 converter.
Here we are talking about saving space, so that's mostly on mobile devices. And irrespective of its quality, I don't think one can expect headphones that one carries around every day in one's pocket to last decades. I purchase reasonably good quality ones, but have to replace them every year. Which is also why I am not excited with those becoming even more expensive.
The main grief I have is rather interoperativity. Right now I have a single set of headphones that I can forget on my pocket and that will work on any of the devices I use: ipad when traveling, home laptop, iphone, work desktop. Apple is introducing its own format. Intel is introducing its own. IT in my company will not replace my desktop for another 5 years. I will end up having to use 3 different headphones or have a pocket full of dongles.
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.
It would be suicide for headphone companies to stop producing analog-only headphones, unless they can force everyone to scrap their analog-only sources (it might be possible with consumer toys like smartphones, and they might be able to convince gullible audiophiles if they can find the right marketing gimmick, but it will never happen in the professional world).
And what about all of the analog-only headphones already in use? Quality headphones are not disposable products (they can last for decades when properly maintained), and nobody in their right mind is going to throw them out until every last analog output in the world has been magicked out of existence.