But that could explane even that fact why Oda chose Ace as son of Roger, case for me now Rocks is actually the one who should be called as Pirate King.
Mihawk is WSS because of what? How Younko became strongest? Because they fight against everyone and everything, they have fight against strongest! Now what about Mihawk? Why every one knew that he is WSS, while he never fight against strongest? Answer is simple: because he was Shichibukai, and WG just made his name as famous, to make every one believe that they got WSS on their side. That's why WG easily dropped Mihawk into the garbage, case he never was someone that is really important to WG, because they always could make exception for one Pirate. Gorosei could just send CP 0 to riverie, to tell kings of World made exception to one most powerful weapon of WG- Shichibukai Dracule Mihawk, which never happened! Instead they send seven worships and they strongly believe that this would be enough to put end to the legend.