I think with Airplay, the market for dock with audio out has gotten a lot smaller.
Think about it. The nice thing about a portable device like the iPhone is that you can pick it up and play with it while listening to music. I think this describe the major of the population. With Airplay, you can walk around the room with your phone in your hand or in your pocket or on the couch, and be able to pick it up to change the playlist or switch to a song, answer a text, a call, etc. It's a lot more cumbersome it the phone has to be docked for the music to be played through the connected speakers.
And if you want to plug your headphone in, it's a lot more convenient to plug it into the phone so you can be mobile, rather than having to tie yourself to the dock with a headphone out.
For those who want a dock with a built in DAC, you probably won't expect a good DAC from an accessory company like Belkin, so the price is going to be pretty high (hundreds of dollars). I am sure someone will market one, but it won't be popular as most people won't spend that kind of money on a DAC to listen to their compressed tunes. Furthermore, if they are fanatic about their music collection, chances are they would use a different player other than the iPhone for playback because of low storage space, hence a Lightning dock with audio out won't interest them.