Is the DAC inside the current iPhone which provides signal to the 3.5mm jack crappy then? In which case, your complaint is moot. The DAC will be in the phone, as it currently is for the 3.5mm output, only outputting to the lightning socket.
No, it's actually not. They use a Wolfson (now Cirrus Logic), DAC which is a very decent chip and the iPhone 6 models have been acclaimed by audiophiles for the excellent sound quality:
http://www.kenrockwell.com/apple/iphone-6-plus.htm#measurements
http://www.anandtech.com/show/8554/the-iphone-6-review/11
However, the 6S has had audio quality issues:
Nevertheless, my headphones are not as sensitive as the ones provided by this reviewer (but they are still offer 108dB sensitivity), and I can't hear any noise.
The point I'm trying to make is I would question if the DAC's used in an adapter would be of the same quality. They would be making less so therefore the unit price would go up.
On top of that, I would have to have an adapter to use my headphones which I find irritating and I'm sure many others would too.