Are you assuming it's a small number? It seems reasonable to assume that without WordPress we might have another 10,000 web designers. Or 100,000. Or 0. Or -10,000 if WP theme design increased the size of the web industry. We don't know.

These sorts of notionally statistical questions breakdown when you try to use "common sense" or logic-but-without-data to answer them. We have no way of knowing what might have happened without WordPress.