Law is irrelevant here, he might use a sword but his DF is still the source of 80+ perecent of his power, so he is not in the same pure sword skill category as Zoro and Mihawk are. Also, pirate leaders of the generation have nothing to do with WSS, Mihawk himself, while being considered a legedary pirate, was never considered a leader of the pirate generation like WB, BM, Roger, Rocks etc. Zoro only became a pirate so he can achieve his goal of becoming a WSS, he doesn't really care about pirate stuff. Both Zoro and Mihawk are swordsmen first and pirates second. So Law can stay stronger than Zoro but Zoro will still be the WSS.