The claim is not really about "motivating" people to create, but making it easier to make a living off their works. People may well create new technology and art for other motivations, but ask yourself: how do they make a living, and would this income be likely if there was no way of protecting their works (whether the protection is enforced by themselves or their employers)? If they don't care about making a living out of their works, it's just a hobby, but the world is not run on hobbies.

There are studies on these kinds of things. The picture is quite complex for patent systems, where economists struggle to prove whether we're better off with them or we'd be better off without them. There is no such question about the need for a copyright system though -- most studies are more about how much copyright protection is enough.