That is just a guess. The Lightning spec is proprietary and reassigns pins at connection time - there is nothing stopping Apple from amending the spec (since it's theirs) to run analog audio signals over one or more of the pins when connected to new Lightning-specific headphones or a Lightning-to-3.5mm adapter (with the accessory simply needing to request this behavior from the phone). Will they? Don't know, we'll find out in a few weeks. But you can't declare with certainty that a second, external, DAC is required.
This is another guess. A bitterly cynical one at that.
I think that's the least likely option, because it doesn't benefit the customer at all, and once they go down that path it will be hard to justify going back. Also, they likely would have already done it with the 30-pin to Lightning adapter and Lightning docks if they were going to do it.
I also don't understand why anyone thinks two dacs is a problem. Every BT headphone has a DAC now and leaves the iPhone DAC unused when sound is transmitted that way. Also, it might actually be a good thing having the iPhone send alerts via the internal DAC, when streaming music to a BT external DAC where you wouldn't want a lot of super amplified ringtones and alert sounds. Plugging the iPhone into a TV uses an external DAC, leaving the iPhone's idle. Etc.
Don't get sucked into the argument that two dacs is somehow stupid. That's not the point, and never has been, but it makes a good strawman for people arguing against the removal of the headphone jack.
Now one place they might implement analogue with a revised MFi spec when the jack is dropped is in the area of using Lightning to pass through analogue audio from an analogue source, like a home stereo, to a Lightning equipped headphone. There really isn't any use of Lightning in that way until possibly now. It really doesn't make much sense for a digital source converted to analogue and output via a 3.5mm to then be re-encoded to digital, just to get it back into a Lightning equipped headphones, where it will be reconverted to analogue for the transducers. It makes far more sense for Lightning as a smart connector to sense an incoming analogue signal via an inexpensive 3.5mm to lightning adapter, and bypass the headphones' internal DAC, sending the analogue signal on to the headphones. It also encourages the upgrade to Lightning headphones, because the adapters will be much simpler and less expensive, making having a drawer full of them affordable.
It would also be useful for using on an older Mac until someone upgrades to a new Mac with a Lightning port. And Apple might include that adapter in the box, so the new Lightning earbuds included in the box can be used with Legacy equipment like an older Mac or iPod.