See here's why you're objectively wrong:
Dashes have nothing to do with the actual movement speed of a champion. Correct me if I'm wrong, but like 90% of dashes are fixed distance no matter what.
Pragmatically, a champion's movement speed is what allows them to get into position and is a constant variable, whereas dashes usually cost a resource and ALWAYS cost a cooldown. And this allows you to slow an enemy and even if they dash, you can still catch up to them and mawl them, because they are gated by a cooldown before they can dash again. And by the time the cooldown is over, they are far too slow to get into position to do anything.
Secondly, it's not up to you or anyone for that matter to dictate how dashes are used. You're given a tool, and you make what you need of it when you play. They are not ment to either go in or go out; they have numerous uses, not the ones you just want them to have.
Let's bring up and example: you play Nasus and XinZhao comes to gank. He dashes with his E and blows everything on you, but you're Nasus, so you Wither him and his movement speed slows down. He's losing the trade and starts falling back but his speed is crippled and you dunk his ass because DESPITE YOU BEING UNABLE TO DO ANYTHING ABOUT HIM COMING IN, you kicked his ass because he had no way of going back out.
It's not the dash you take advantage off, it's the window of time between the dash's target that you're exploiting; the enemy is far too crippled to get into a position good enough to dash again. Even a Leblanc can be crippled and shit-on. Yasuo is strong for different reasons and not so much to his THEORETICALLY infinite mobility. He can dash for as long as there are enemy units nearby, and to get from one enemy unit to another he needs to walk there, good luck doing that when you're 90% slowed.
There's only one champion in the game with legitimately conserning mobility, Kalista. And despite that, you can still cripple her because her mobility is gated by her attack speed and movement speed. Both of which can be crippled with abilities AND items, many of which are built to synergize for that reason.
Lastly, the most absurd thing I read was this: "mobile champs get a hidden strength that not many even consider. They are almost immune to slows in close combat". This is stupendously wrong. Literally all the hypermobile champions in the game are squishy as all fucking hell, and can be EXTREMELY punished for making mistakes and positioning badly. Immobile mages on the other hand, might lack escapes, but they have the power and CC to punish whoever does the wrong thing at the wrong time.
It's up the the player to utilize the tools they have to make the best out of each situation, and that requires game knowledge, nothing to do with slows and dashes. What you should worry about is mobility with untargetability because untargetability practically renders your slow ineffective, but only 3 champions have this
.