There is absolutely no need for an external DAC in the headphones.
The DACs in the iPhone/iPod are already very high quality, and there's nothing wrong with the 3.5mm jack.
It's
possible that they want to replace the 3.5mm jack with a lightning connector because it's thinner, but that seems unlikely.
Here's a thought though: what if it's to enable noise cancelling headphones that are powered by the device?
I love my Bose QC20i, but I hate the separate battery/control module attached to the cable:
Use the phone to power it, perform the DSP, and automatically disable noise cancellation when the headphones are disconnected, or connected with nothing playing.
It is useful to have the option of enabling the noise cancellation with nothing connected to the headphones if you just want peace & quiet, but you could still have an option on the phone to do this.
However, there have been a number of times where I've put the headphones away and forgot to switch them off. So the next time I go to use them, the battery is dead.
If that could be built into the phone, it would absolutely be worth the proprietary lock-in.
Sony have something similar with the Xperia Z2, but I can see why Apple would prefer to use the Lightning connector for it.