You have failed to account for the part where yas's shield is completely free. every other shield in the game costs mana, and you can effectively whittle your opponents mana bar down with trading. Yas, on the other hand, gets a completely resourcesless shield.
So, lets compare his shield to the other essentially resourceless shields in the game: Face of the mountain (90 second cooldown) Archangels staff (120 second cooldown) steraks gage/Maw (60 second cooldown).
His shield is, by far, statistically overpowered.
We can do more math. laning phase can take anywhere between 7 to 15 minutes. lets just say its 75 for easy math. In 7.5 minutes, yasuo can block 26 trades with his shield, entirely for free. for 15, double that, 52 trades.
compare to a janna, who also has a shield but who has to deal with mana. Base mana pool 0f 841, 70 mana cost for a shield (that does 85 damage mitigation), If all she does for the first 9 levels of the game is shield in trades, she can do between 12 and 24 shields during a laning phase (rough extimate to account for mana regen/5s).
You might be like "Aha! I have you now, 24 and 26 is close in a laning phase, hold my intransigence while I wallow in self-righteousness" but you forget that janna, in doing so, has forgone any possibility to offensive trading with her kit.
Yas, on the other hand, gets to get his free shields while offensively trading twice every minion wave.
That shit is hands down, broken.
dropping his cooldown to 30 seconds means he can effectively get 15 -30 free shields in a laning phase, and any mistake made during a trade gives his lane opponent an oportunity to deny him CS. Its still, statistically speaking, broken as shit given his 30 second cooldown ultimate at level 16, but it would make laning against yasuo feasible.