BB is already a Yonkou. That was his primary goal from the moment WB died. He went around to all the places that were under WB's influence, and put his flag on them. It's not like he stole any other Yonkou's territory or else they would have declared war on him already. Also, being the Worlds Strongest Man doesn't = PK. WB held that title for ages, yet never took the PK title (although he didn't really want to), and as shown, just cos your the Worlds Strongest, doesn't make you undefeatable.
It kinda does,thats what the title means,at least if we're talking about 1 vs 1 fights.If you're not unbeatable,you're not the WSM.