hfly
What you are saying is true and well known. I'm trying to raise a question about why, for more than 20 years, TK hasn't added an overnight flight IST-NYC. Why would TK forfeit this golden goose? It could be any one of the reasons you mentioned, but in a 20-year period, I'm sure TK would have overcome any issue if they wanted to.

My theory after looking into his matter is that TK has (or had, now that there's TK111) a misconception.
All major European carriers (BA, IB, TP, AF, KL, LH, OS, LO, LX, IZ, etc.) only have daytime flights to NYC for the good reason of a/c utilization. TK copied that behavior.
On the other hand,TK may not see other carriers that do have overnight flights as direct competitors: LY is not a connecting carrier, and QR, EK, or EY are too far away for most European customers.

Since time-wise TK resembles LY more than European carriers, I wish that they overcome their misconception (if such one does really exist) and see the obvious: an overnight flight is what customers demand.

Unless we have an official TK representative answer this question, we will never know the real reason why TK still doesn't have a regular overnight flight to NYC.