Players aren't picking him because he's flashy. They are picking him because of an overloaded kit which gives a clear advantage over the enemy once you practice playing him.
And explain how he's not killing variety. He's the most contested pick in pro play. I guess that's fine, maybe he is a pick that requires skill and coordination to shin... oh wait, he has more than double pick rate than second most popular support and more than combined pick rates of bottom 16 (!) support champions. He is seeing more play by himself than more than half of the support roster combined does, for crying out loud.
And that is a good question. If I'm the last pick would i pick Janna to peel, Sona to poke or Leona to engage... Or Thresh to peel, poke and engage at the same time?