millions of pirates want to be PK. To do that you have to get through the Yonko. They are at the absolute top of the field. No one cares about WSS.
The Yonko are constantly tested not only by pirates but by marines. Mihawk was safe in the comfort of being a Wwarlord or he would have been worried about getting magma fisted. Why do you think he was hiding out in East Blue chasing fodder? Because in the NW you are either under a Yonko or fighting them. If he went to the NW he would either be a sweet commander or a homie. He would either be a Calamity or dead etc.
So no one wants to be Yonko, thanks for proving my point.
And no you don't have to go through Yonko to be PK, Roger sneaked into one emperor's territory and didn't defeat any Yonko or admiral, it is only Luffy wants to defeat them all.
Also that would make Shanks only as important as Kizaru, Fujitora, Green bull, Kaido, Big Mom and BB. So Shanks who is already weaker than Mihawk, shares importance to Fujitora who is also weaker than Mihawk, for someone who wants to become PK.
People all around the world who are swordsman who want to be as strong as they possibly can be, AUTOMATICALLY want Mihawk, because unlike PK, you don't have to take specific route, all it takes to get Mihawk's title is to reach the pinnacle of strength as a swordsman, something one armed Has Been obviously couldn't do.
And funnily enough, NOT A SINGLE SOUL gives two shits about Shanks' title.