The problem with flash is the players.

I've been playing games replacing flash for other spells like teleport or exhaust, but in almost half of them i'm kiked from the team builder room, there was even a time some dude trash talked and complained because i had ghost instead of flash. And guess what, my games are going much better with exhaust/teleport/ignite (especially top) than when i had flash, much cleaner kills and counters to heavy sustain, not to mention the map presence due to teleport.

Flash is a summoner spell for champions that require positioning, like Amumu and Fid and carries without escapes, like Kog and Twitch.

Going to mumble some nonsense saying how all pros use flash? Imagine this scene, you are playing as Vayne, you were caught in a wall-flashable pincer trap by Graves and Riven, all late game full build. Which do you think has a better chance of survival, heal/flash or heal/exhaust? Or even exhaust/cleanse.