While I agree with your main point, I think UA has a lot easier prospects here than DL would, given that UA fliers can connect via *A to TK flights in to the ME and Africa (and even Asia if they are bargain hunting). Furthermore, there's obviously going to be more O&D traffic between NY metro and IST metro than ATL (given that both airlines have huge connecting networks feeding in to those cities I'm considering that neutral between the two). The Turkish diaspora in NY is huge. In ATL, not so much.

I fly NA to IST quite often - it is my second most frequent international route after NA to LHR. I've made the journey in excess of 100 times over the last 14 years. What you point out about the TK of years ago is quite true. TK today, on the other hand, is awesome. I would take TK over BA LHR-IST and certainly take TK in J over DL or UA NA-IST. TK's food and service are superior, the CIP handling at IST is ideal, and the hard products are, at worst, equal between the aforementioned carriers.