And to add, Mihawk said that PK was a harder goal than becoming the strongest/best swordsman.
Seems to me, for both The possible next PK and the next possible S/B Swordsman, at that time, It would been hella useful to to face each other, even constantly to constantly improve. Even if Shanks might not see himself as a swordsman, it might have been a greater challenge if he restricted himself to swordsmanship. You say used in battle, but from what we've been shown, they were squirmishes we see for a single move. Squirmishes between highly powerful people, undoubtedly, but not necessarily battles.
Mihawk now refuses to fight Shanks, so we don't know, and as Vorlianis pointed out all we've really seen him do is draw his sword, a couple of times.
I think his point was more so that just because they wield or carry swords doesn't mean they consider themselves swordsman. Yosaku and Johnny wield swords but I don't know if they could be considered swordsman. For that matter, that guy from Shanks crew Ben Beckman carried a gun, but used it like a club against the mountain bandits. Possession of the gun does not automatically consider him a gunslinger.
As for the actual topic, Think WB might have unintentionally, but well-deservedly, earned that nickname because of his reputation for strength, power, and potential grew, and spread, along with people challenging him but getting defeated, until there were no more challengers.
I do believe that some idiot like the Sh's imposters will try to take the WSM title by claiming it for himself. Don't think pirates in the OPU can let things just go like that. They will mostly clamor to replace the weakness prone former anything. But I think they would then constantly be forced to defend it, until it was lost, and so on and so on.Think there could be a constant series of challenges until some1 suitably strong gets the title and word of their reputation spreads, and then something similar to what I stated above about WB will happen as well. But I think the process to find some1 appropriate for the title for that time period will take years. Think it will be a generational or bi-generational process, where as long as pirates exist in the OPU, then there will be certain very specific goals for large amounts of pirates, that these next generation pirates will eventually in some way or another usurp from the last generation.