I agree, in fact I posted this same solution several weeks ago in the previous "outrage" thread concerned with the removal of the analog headphone port.
Just because Apple hasn't implemented this in the past doesn't mean that they couldn't in the future, on the next iPhone.
While it's possible to do, if Apple were going to allow analogue signals over Lightning, they would have done it in the transition to Lightning with the 30-pin Lightning adapters. But they did not.
All passing an analogue signal through Lightning does is require an adapter that changes the shape of the plug, and that's the worst insult possible for the customer. Apple learned that the hard way with the original iPhone. If Apple is going to do this, then they have to offer some benefit to the customer in exchange for the inconvenience, even if it costs a bit more in the beginning. Also, it does nothing to improve Apple's bottom line as they will have to continue to source the same high quality DAC & amp, rather than take the savings by offering a lower quality chipset needed for the built-in speakers.
But ultimately, they end up encouraging those with legacy 3.5mm equipment to continue using it, which does nothing to bolster demand for digital audio products, thereby eliminating incentive, competition, innovation, and price reductions on a market which currently has an expensive entry point.