Pretty much this.
The professional scene meta is pretty strange to me. It's like there's about 3 useable champs per lane, and the top one or two get banned in every game, so the 3rd one is actually the most commonly picked. Elise is drawing bans now (finally) so we're going back to the two most "utility" junglers in the game Rek'Sai and Gragas. In fact I'd almost make a bet that in any game for the rest of worlds in which Elise is banned and no other junglers are banned, the two that are picked WILL BE Rek'Sai and Gragas, and that's it. If Elise doesn't get banned, then we'll see Elise and either Rek'Sai or Gragas, and those are the only 3 junglers we'll see for the rest of worlds, unless more than 1 gets banned in a single game.
The fact that even with a potential game breaking bug Gragas is still a "must pick" is proof that he's too strong. Not because he does massive damage, or is exceptionally mobile, or is overpowering, or makes plays constantly, or has a huge effect on the game, but because his utility is simply 2nd best among junglers in the entire game, and that's priceless to a well-structured team. Once or twice during the game, no matter what is happening Gragas WILL make a huge play, and the constant threat of that happening will make sure the game goes calmly and predictably for at least one block of time. Gragas is like the hype-stopper, or the anti-ham (maybe he ate it all?) It's like nothing is going to happen until Gragas starts a fight, gets picked, or uses his ult, and that's true for both teams as long as he's in the game. These two junglers are simply the two that fit the best into any and therefore every team comp because no synergy is required for them to be good. Even after multiple nerfs they'll never go away. Riot of late has been pretty hard on champions that encourage a "binary" playstyle, but champions who can do everything are always going to dominate the pro scene and put viewers to sleep. (
to name a few.)