Why was Yasuo not given a Dash limit?
Give a Yasuo 5 or so of your minions to dash with, and you will NEVER pin him down if you're not Ryze or Malzahar. Granted, you can't dash to marked targets, but if there's enough stuff to dash to, that makes no difference.
Can't you do something like this?
Sweeping Blade requires a Dash Charge to use. If you dash to an enemy Champion, the charge is refunded. You receive new charges every X seconds, up to a cap.
Why this? If you dash to too many minions, you run out of dashes.
Why bother? Yasuo's infinite dashing makes him very frustrating to pin down, like a Lee Sin on steroids.