Most American airlines break guitars
He is probably right that United has designed a system to discourage claim, but also likely to be a result of an incompetent process system. More likely, both of the above.
Would this hurt United? In the short term, yes. But in reality, we know that all US-based airlines are the same. I had my lugguage broken and compensation/repair refused by American Airline a few times, flying F class as an Executive Platinum member.
Asian and European Airlines tend to deal with these situations a lot better. SQ was the best in my experience. This is a direct effect of American protectionism. Not just the lugguage handling, but general indifference and lack of accountability as happened when he first complained to the crew member.
This musician may trigger some short term response from the industry, but there is likely to be no long term impact for domestic US routes, because that is the industry norm -- unless foreign competition is allowed into this space.
p.s. some follow up and comments here.