idk about not that durable since he basically has multiple lives,
how so?
The masks exist canonically in-world. They're magical charms that protect the Knight's body (which you can see the need for because there's instance of what happens when a Vessel loses its body in the Abyss and the endings). So there's reason to believe the magics guarding his body would have to be broken before he can die.
I wiki'd hollow knight, and him dying seems like it would be a disadvantage in the fight because of having to fight his shadow or whatever.
We wouldn't allow him to do anything after his body is shattered because he turns into a literal god and savagely wrecks an eldritch horror with ease when he does. I'll add that to the writeup for tonight. ---