I fly IST - DTW several times per year due to working in Turkey, and got so frustrated that I completely changed to SkyTeam. I tried flying a combination of UA - LH, but with LH they still try to force you to fly SunExpress (disgusting) so I decided that it would be best to make a clean switch. I agree with all the points you listed above. TK works best for the occasional traveler - someone who doesn't care about any FFP aspects; who needs a direct flight from Turkey to their home country (due to convenience/transit visa issues); in general someone who flies basic economy and just cares about the 2 free checked bags and the decent food in Y. Sorry if it sounds harsh, but that's been my experience. They don't seem to care about frequent, discerning, serious flyers at all, in any aspect. Customer service on the ground is pretty much either nonexistent or incredibly rude and condescending, sometimes making up outlandish lies to cover their incompetence - with a typical "won't care, too bad not my problem" attitude. Add in the frequent IT glitches on both their mobile app and online, and one can easily have a nightmare on their hands. It doesn't help that TK is a government-controlled flag carrier for the country and faces virtually no competition on many of their routes, both domestic and international. A monopoly like that is never good for the customer.

So much for the vaunted "Turkish hospitality" which they are otherwise known for (which can be found in luxury hotels for example). Among other aspects, it's typically a cultural thing, as with some other places and airlines as well. It's a shame because Turkey is a wonderful touristic destination, and the local people you meet can be incredibly friendly and warm. Now, I only fly TK on domestic routes, since SunExpress, Pegasus, AnadoluJet, and the other LCCs are even worse.