The one argument I never have understood in this game
Is that
helps immobile champs more than mobile ones when we have people like
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who use it to basically jump immobile champs before they can fight back.
To me it actually benefits the mobile champs more because it takes away from the power budget in an immobile champ. Every mobile champ basically uses flash with some exceptions and often to dive immobile champs because they don't have the power to fight back because if they did
on immobile champs would make them oppressive. See
. She has short range but flash completely negates her weakness.
The game would actually be easier to balance without flash instead of going "Oh, but flash is more effective on immobile champs than mobile ones". Because let's be honest that isn't true. Flash derives its strength from a kit as a whole not your mobility. It seems more useful on immobile champs but that's because its more noticeable but it causes the same problems with mobile champs.
at least has the counter play of letting you react to it. (on specific people its actually better than flash, but situationally)
In almost every situation where LeBlanc could flash... she's actually better off using her W. Riven usually doesn't need flash to close on an opponent or to escape opponents. The situations where it really benefits her are usually tower dives or surprise (flash stun) both of which are situations that benefit immobile champions at least as much (perhaps more).
Let's say you play a game and (oops!) you forgot to take flash. If you are Lucian then you have dash to fall back on that can keep you relatively safe in lane or allow you to close gaps to get kills. On the other hand, if you happened to be a classic immobile carry such as Jinx, you are in trouble the moment your gap is closed. So you would be forced to play much safer if you didn't have flash.
Darius is an immobile champion who is easily kited. Casters at worlds have pointed this out as a major weakness for Darius. If Darius wants to be aggressive, he needs to close the gap. And that means that he benefits massively from having flash. The difference here is stark since mobile champions can engage much more easily without flash, but Darius practically needs it or he can't aggress effectively against intelligent opponents.
Ashe is immobile, but has a lot of CC that can help play safe. She is an example of an immobile champion that can do better if she happens not to have flash then other champions that are immobile and happen not to have flash. That said, jumping walls can be very important for any champion to do and the fact that she can't means that she benefits more from flash than champions that can already jump walls.
What this all comes down to is that mobile champions already have play making options in their kits and immobile champions often don't. And this is why immobile champions benefit greatly from flash... more so than mobile champions.
You might want to consider what it means to be a mobile champion more carefully. For example you place Udyr on a list of mobile champions. Udyr doesn't actually have a gap closer. So his mobility is tied to increasing his base movement speed. So actually it is Udyr's lack of a gap close that causes flash to be effective on him for closing the gap with an immobile champion.
This entire discussion is just a bit more complicated then everything said so far. Ghost provides a lot of mobility but the benefit of flash is surprise, play making, and instant escape. Whether or not you are considered a mobile champion or an immobile champion flash creates an instant shift in the zones of control. The reason flash benefits immobile champions more is that immobile champions don't have a means for quickly shifting their zone of control but immobile champions already do this to a certain extent already.
An argument to say immobile champions benefit more from flash than immobile champions amounts to mobile champions who decided that they don't need to take flash at all. In this case, they "benefit" by virtue of freeing up a summoner spell that immobile champions consider a must have. But that also suggests that mobile champions are intrinsically the same as immobile champions sans their mobility and this is not the case, because immobile champions actually tend to benefit from their lack of mobility be having more in their kits for other types of abilities. So intrinsiclly, sans mobility, immobile champions tend to have an edge. What this amounts to is that mobile champions as a whole can't benefit more from flash than immobile champions.
and get caught out of position or pincered, I'm screwed UNLESS Flash is up. And that's kind of an acceptable trade-off, because I can escape once every 5 minutes and I probably shouldn't be out of position more often than that or I deserve to die. I think of it like an Anivia egg -- I hopefully won't need to use it more often than it comes up if I'm playing well.
gets caught out of position, they're screwed unless their mobility skill is up -- but it often is, or at least it often comes up during a fight.

has a near permanent 60% speed boost and when you add Ludens echo, that girl can move at the speed of light.
master of the infinite bush hopping. I also forgot about
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would be if flash didn't exist. Immobile laners would be completely unplayble if champs like these were ever on the enemy team.