What is the relationship between Houston and Turkey? What drives that traffic vs DFW?
Business first and foremost; Turkey has a very active Consulate General in Houston (though to be fair Houston has nearly all the consulates in Texas) so just take a look at all the business events they cater to in the area. There is the diaspora as well, though certainly not enough to drive a flight without the business, but it is there and active. Plenty of other Euro-VFR traffic also; example, plenty of my Italian countrymen in the Houston area fly TK for the low fares to Italy, even with the massive backtrack. $~600 roundtrips to Italy for a family of 3-5 makes it worth the backtrack to them (not to me, I stick with FRA/MUC).
The folks mentioning Austin are also astute; while Austin has grown and the new non-stop flights US has help keep (and generate more local traffic), plenty of Austin metro folks drive to IAH to catch direct flights on IAH's international carriers. Even people travel from SAT and DFW to IAH to catch said flights (EVA even runs a bus service from Dallas for their Houston flight.).
There is cargo and the big connecting corporate contract TK has with BMC software and Sysco. I am not looking to list everything, mind you, but the point is there is certainly a market for TK at IAH.
That does not mean they might not go to DFW also, but I see no logical reason that would cause the closure of IAH. They only challenge would be political IMO if relations worsen so much causing cuts to the USA, and we are a long way from that...but what do I know LOL.
“Without seeing Sicily it is impossible to understand Italy.Sicily is the key of everything.”-Goethe "Journey to Italy"