How do you demonstrate it either way? If doing things that make us happy in life doesn't make us better people, what does? I'm pretty sure that whomever first said "marital arts makes us better people" didn't simply mean the act of punching, kicking and throwing makes us better people. It's finding something that we enjoy doing, maybe even passionately and hopefully with people we also enjoy, that brings us happiness and ultimately makes us better people. While martial arts may not be any better at that than any other activity, it certainly isn't any worse either.
Great, so you're stating a belief rather than demonstrating anything.