Put me in the camp of "requires natural ability" + other stuff. I am surprised, shocked even that some people don't think natural ability is required to do things which only a small percentage of the population can do. That being said, I bet there are a lot more people who COULD be tour pros (or insert other upper echelon job in any field) based on their natural ability. BUT, because they don't have the other stuff, such as desire, mental aptitude, work ethic, etc. they aren't. I would also bet there are more people who have the desire and work ethic but lack the natural talent to become a tour pro or whatever.
All that being said, I don't think it's a good reason for someone to not follow their dreams if they want to. More power to 'em.