I actually like the idea in theory. It's like ethereal weapons from Diablo 2. But this weapon is way over the top, obviously. An unrepairable weapon should be pretty good, but not absurd. Maybe good stats, a +skill, and slightly higher than normal damage. I'm not sure if the weapon would last long enough to be naturally upgraded anyway, though, assuming you don't bash chests with it. It might also need something like half the natural durability to really become a situational weapon.
