Let's say I'm Fiora. I have 300 AD. I'm in a top lane bush. You come near the bush. I ult you and deal almost all your HP in true damage with almost no counterplay at all.
Or I'm Gangplank. I have a barrel and I'm in the bush. I instantly put down a barrel on top of you, detonate it, then auto you to death and keep chaining barrels until you die. You're slowed to kingdom come, so you can't do anything even if you're the tankiest motherf*cker out there.
Hello, I'm Garen. I wait inside bushes to silence you, shred your HP dosn with an ability I don't even have to aim with, then use an ability that nukes you for having missing HP. Doesn't that scream counterplay?
My point is that many champions can pull off that scenario better. It's advantageous, and it's not like Shen has Mundo-levels of damage. Mundo's Q does more %HP damage than Shen's Q in three autos (from full HP, I assume). I don't understand why you are so adamant about keeping a tank irrelevent.
EDIT: Just replied to the wrong person. Well played, mobile site.