How is Yasuo not viable? He has a strong, safe laning phase since he can just block projectiles and dash away from ganks, or simply knock them up PLUS he has a shield, his mid game/teamfighting is probably one of the best of any mid laners, and his late game is just instakilling the enemy carry.
I don't know how you play Yasuo, but it's obviously wrong. Yasuo needs a nerf before he gets buffed. He's even been played in the LCS and was just played this Season. He has like, a passive on every thing ability he has and really doesn't have many fall offs. He's squishy, and if you get behind early you'll just have to play more safe until late game. But even then in comparison to other people like Syndra if they get behind, they have to farm a lot and actually try to farm. Yasuo's wave clear is stupid and you already have more CS than everyone else in the game even if you're going 0-5.
Yasuo isn't unviable, and really I would be totally okay with nerfing him right now rather than buffing him. He's really hard to play, but a Yasuo main just stomps the game like it's nothing because his kit is designed to basically invalidate more people than a 10-0 Darius.