People have been talking about this since season 1. There used to be constant calls to remove flash. Because it is such a high-impact spell though, the entire game has been balanced around it, and without it many champs would need major adjustments. They also can't make the other summoners powerful enough to compete with flash without needing balance changes throughout the game. Just look at what happened when challenging smite became good enough that a small group of champions were willing to take it instead of flash: it was immediately nerfed. It's not a perfect parallel, because cinderhulk was a big part of that equation as well, but two of the enhanced smites have been nerfed this season despite the fact that neither had the impact of flash. Flash is unbalanced, but it has been so unbalanced for so long and the whole game has been built on that imbalance, and removing or changing it would cause a domino effect.