"Let's just throw more mobility at the problem"
It feels like, recently, Riot's solution to balance problems has frequently been to just throw more mobility at it.
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too strong? Nerf his range but give him way more mobility!
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losing DFG? Give her a huge MS boost when she uses Q!
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bursting too fast? Nerf her damage, massively reduce the cooldown on shunpo!
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having trouble shortly after launch? Remove the max range on his dash!
That's on top of all the recently released champions like:
"So how’s he different? Well, Mini Gnar’s one of the most mobile champions in the game." -Gypsylord
"Rek'Sai's a unique champion with... piles of strategic mobility options" -5.1 patch notes
Martial Poise. Free dashes that have the same cooldown as your attack speed as long as you are in range of someone. Don't really need to say anything else.
Riot says they are aware of mobility creep, and they think it's a problem. They keep saying this, but they also keep injecting more and more mobility into the game.
I've recently started playing Sion. I'd say that, in about 50% of my games, I have almost no chance of ever interacting with the enemy carries. There is no way I'm ever going to land a spell on Kalista or Leblanc unless they screw up horrifically, or someone else CC's them first.
I trust that Riot understands the game better than I do, and that they put a lot of thought into every major champion release or kit rework they make. At the same time, I'm a little concerned by the exponential increase in mobility recently. In S3, people would have laughed if you had told them that Riot would ever release a champion with a dash on a sub-1-second cooldown. Now we have Yasuo and Kalista.
Maybe it's that players find high mobility kits more fun. That wouldn't surprise me at all. But it's starting to have pretty serious consequences. Point and click CC has been getting systematically phased out at the same time that mobility has been exploding. This is a double whammy that makes it incredibly difficult to interact with certain ultra-high mobility champs (like Kalista or Leblanc).

or trash level of either
(no mobility), then they are OP/UP.
All in all, mobility is not a huge issue if they keep champs in a relatively fair range. (too much mobility would give too much safety, too little would be 