I don't know if the current users of iPhone will take this as a number of them are music producers (or enthusiasts) and their current very expensive hardware won't be compatible with the new iPhones.
The old to new magsafe adapter is only $10, so maybe I'm being too hard on them. On the other hand, their Lightning to HDMI or VGA adapters currently go for $50.
Bluetooth earbuds would be a pretty hard sell if they come out and say "We're changing to Lightning for better audio quality. Here, have some bluetooth earbuds with worse audio than your old wired ones."
EDIT: How small can you make a decent DAC anyway? All the audio ones I've seen are way too big to put on an adapter cable, but that might be driven by the optical and RCA connectors more than the conversion hardware.