Agreed with your first 2 points (UA should and will provide some comp), but your 3rd point is overblown, yes?

My point in this thread is that people should take some responsibility for themselves. The rules clearly say you cannot plan to bring a large item like this on board. You might get away with it, but you must plan for the situation where you do not. And that means having a case for the guitar that will stand up to normal airline handling.

If that guitar case was appropriate for airline checked baggage, and it was nevertheless abused and damaged, then UA is 100% at fault. I see no blame on the passenger.

If the case was inappropriate for airline baggage (which it appears to my eye in the pictures, based on similar cases I have owned and seen), then the passenger shares SOME blame.