I would agree with the OP, I have found everything perfectly playable at 4k, provided you don't want any AA, (which I personally think you can do without when you have such a high resolution), and are willing to drop a few settings.
Fallout 4 was spot on, if you had no AA and put the god rays on low. (everything else high/ultra)
Likewise arkham knight, I ran with no AA and all the gameworks turned off and it ran flawlessly, despite what people may say about the performance of that particular game.
Note that I always turn of any motion blur or stupid DoF effects which probably helps a lot.
Anyway, the way I see it, if you find 50-60 fps acceptable the 980ti is great, it also depends largely on what kind of games you like.
Over the next few months, all the games that I want to play (Xcom2 and Constructor HD) aren't going to push my card too hard.
Whilst I would like to upgrade when the next gen comes around, the performance increase needs to be 50-75% for the same price for me to really consider it.