...but 
has been a "must have" summoner for YEARS (with a few exceptions like Hec or Olaf etc). Being a must have is an indicator of being broken, not balanced. If any champion was 100% pick/ban for even a couple weeks theyd be toned back....yet here we are years later and Flash is still the reason we actually only get to pick 1 summoner spell every game.
They nerfed flash many times when the game first started out which was justified. At this point, however, flash is such an integral part of the game that gutting it or removing it would be like removing Baron from the game. Flash isn't broken or abused in any way so there's really no reason to nerf it or get rid of it. Teleport currently provides way too much safety because you can die or lose a trade and then go back to base and TP back immediately without getting punished at all. Everyone is taking it for that reason which is why it is being nerfed not because it is mandatory.
I pick summoners based on enemy team comp, while I'm not great at it, I've seen enough personal evidence you don't always need flash. Flash is good against junglers with natural gap closers, most lane matchups can actually be won without flash, but with the exception of adc(and commonly their support), that I would agree should always take flash.
If you cannot win a game without flash, it might mean you trade poorly during lane. (Again with exception of adc, even a good one would probably want flash...but I'm not really sure if it's mandatory there either.)
"Everyone taking flash" is an older meta choice than adc bot lane, it has many flaws and exceptions to the rule.
Top lane used to 'always need teleport', and with the advent of many ignite early game top laners facing people with teleport, I've found that mid having teleport can counter the top teleport rotations as a death sentence to them after they have already died to ignite in lane, and so my philosophy that "top should always have teleport" has become "try to have as many teleports as the enemy team, across your team, but it's not always necessary". These colloquial 'everyone should do this/that' ideas are only generally correct, and often why people fail to struggle to emulate pro builds is the same reason 'you should always take flash' is sort of wrong.
Ghost/ignite feels strong too, because of their extremely short cooldowns in comparison to a teleport or flash. With celerity s8 stuff, it can be responsible for a good chunk of AP/AD and extra damage, while also offering some escape potential, and almost twice as often.
Cleanse is also super powerful, and could very easily replace flash in certain matchups where it's just not considered. If you are going to die to cc cheese, or ignite cheese, or cheese in general, Flash is taken as a general purpose aggressive spell, but also protection from cheese, and in many cases cleanse can protect you from this cheese way better sometimes.
It's only because Flash is multiple purpose and general in the first place it's always selected, people are comfortable using flash, and always have this is more weighted in their choice to take flash, than what is actually optimal. People that drop flash, practise that, they don't just drop flash and are pro without flash in 1 game, it takes time to get used to not trading super greedy just because your flash is up, and you actually become better in lane phase overtime without it once you use it again anyway.
Flash is the most common summoner spell that every champion is balanced around.
Alright try playing 
in diamond+ without flash and then say that again with a straight face.
