Are there any studies showing that copyright actually does achieve its goal of promoting the creation of art? Can we show a causal link between copyright and art creation? Can we show a proportional effect where longer copyright terms lead to more/better art creation? I suppose it would be rather difficult. You'd have to find a control group that's otherwise just like the USA (or your comparable country) but without copyright.
Is copyright really the driving force, without which we wouldn't have creative output? I have my doubts. People were drawing on cave walls and parchments long before copyright. Would people simply not create art if copyright didn't exist? And if so, would we really miss that particular art?