I myself have about a thousand dollars invested in high-end headphones and earbuds (Sony DJ cans, Extreme Isolation EX-29 for the open office, Sennheiser earbuds for the commute) that I have no intention of replacing or buying a dongle for.
You don't need to buy a dongle - it comes with one.
I seriously question the quality of the DAC in that dongle.
Clearly you've never seen any teardowns of Apple adapters. Usually the best (cleanest power, data throughput, etc) in their respective categories.
If you cared about the quality of the DAC, wouldn't you have bought a great one already?
And you think the built in DAC on the old iPhones was better?
That's a key benefit of this change. Ditch the legacy analog port that is wasting space. Allow consumers to use a DAC that meets their quality standards, be it a cheap one in the dongle, one built into headphones with a lightning connector, or an external / improved DAC dongle.
Audio DACs have not been the limiting factor in the analog signal chain for a long time.
There are no bad DACs anymore and there haven't been for a decade. You'll probably be getting slightly better analog environment since the DAC isn't near the antenna and CPU as well!
Why do you think it would be any different from what they built into previous models?