Well its obvious why. Hes a bard not a chirgeon.
Your mistake was believing him 😂
Oh i'm aware that probably nothing good comes out of it. But there is no difference between no eye (after operation) and inserting the magic eye, despite the description saying you can actually see with it and it has a magical aura. Also, the character says "Oh thats better" after inserting it.
Since many other things are broken (E.g. the hags hair, or Loviaters blessing) i assume something should happen/change - whether its a good change or a bad change is another question...