I think it does have a cost to it though, obviously not in mana but in time, max level its an 18 sec CD and yasuo isnt one to get 40% CD, normally anyway, because his Q cd scales with AS. This means usually he gets one windwall in a fight. If anything is low risk high reward on yasuo its his shield passive, I mean no damage for walking?
I mean I'm simply saying Yasuo's wall sucks to deal with in situations but it is counterable, there are ways to work with it. I has a similar function to Fiora's riposte, saves the players life if used responsively. So I'd say it does have a moderate or high cost/risk to it because you only get one and if you waste it early you get bodied later in the fight.