The entire point of the shield is to allow him to channel the dang skill. it's not like the shield is just utility like Karma. He has to be able to stay in a fight. Without being able to mitigate cc, he's going to become a 40% win champ. His entire core is about actually being in the fight.
W doesn't need the Unstoppable debuff to be able to stay in fights. Adding the fact that Ornn's W duration is like less than a second.
I don't see them nerfing Garen for having a kit to be in the fight, even though he heals 3.2% of his max hp per sec at low HP and has up to 97.6% tenacity to get away.... even on a Lethality build.
His healing requires him outside of combat. This also applies to Ornn with Warmogs anyway. Also Garen's kit has 0 scalings with %HP builds like Ornn's does, so it's sensible for Garen to typically go for Lethality builds instead of the normal beefy HP builds.
I don't see them nerfing Mundo's ability to stay in a fight.
Grievous Wounds from items already kills him enough. And he has close to no CC aside his Q to be able to peel a lot for his carries.
I don't see them nerfing Maoki's insane heals to stay in a fight.
His damage reduction ult removal kinda hurts him back during the update. But nonetheless, Grievous Wounds can also hurt him plus his Q has been repeatedly nerfed.
That's the purpose Ornn's W's barrier serves: He can't do anything for 2 seconds, and he's literally the ONLY champ who is subjected to that rule by his own skills. He is basically CCing himself by using W. if you can't avoid his last tick Brittle, you probably shouldn't be playing ranked.
Chaining his QE CC combo and then using his W can seriously destroy his opponents and then uses his shield to mitigate any incoming counter-damage. He has more than enough chained CC to be able to whittle down his opponents with his W. I'm guessing a change in playstyle where using his W first directly at the enemy Riven's face is no more a good idea.