It sounds to me like they don't want to cover it because this is "normal wear and tear" and that there is no minimum number of rounds to wear out those parts. I don't know why they didn't just tell you that, but that's my interpretation.

Even when a product says "lifetime warranty" I take it as a given that if I want it fixed at the factory, I need to spot the defect sooner than later. If I had a gun that worked fine for four years and then a part wears out, I'd consider it my responsibility to get it fixed.