Just because mobile champs pick it doesn't mean no one should be able to take it.
If Flash got removed/nerfed into the ground and unusable, then so should Exhaust, Ignite, Heal(?), and Teleport as those are the most common runes to find secondary.
But that wont happen, because SURPRISE! Not every champion makes use of Ghost, Celerity, Barrier, or Cleanse. Heck I usually only ever see Ghost on Singed and Hecarim, and I rarely see Barrier, Celerity, and Cleanse taken.
Just because a summoner spell seems op doesn't mean it is. Because honestly? Flash is one of the most fail summoners there is. Have you ever flashed into a wall rather than going to the other side while hugging it? Have you seen how many times people flash INTO skillshots? Or into the rest of the enemy team when running away?
As 'strong' as Flash is, it's also a very iffy summoner because as great as it is, it can also put you into even worse situations instead of helping you.