Shanks as part of Roger's crew probably doesnt care much about One Piece or becoming Pirate King. He's been there and done that. All the views we've seen of him and his crew have shown him basically chillin, and like WB I think he simply just enjoys life and hangs around. Kaidou and BM are presumably the sort who want to find One Piece but are not capable (Kaidou was pwned by Shanks just before the Marineford battle).
Now you add BB to the mix and suddenly the 'power balance' in the New World looks precarious. Suddenly you have 3 powerful crews actively after One Piece and only 1 powerful crew who has the capability to keep them in check. Say what you will, the Marines are as scared of Shanks as they were of WB, if not more. On the face of it, the fact that Shanks and crew can match WB's power without DF users, massive armada's and crazy allies should make them an even more formidable enemy. Shanks crew is also at their peak in terms of age and skill unlike WB who had faded with age (and his commanders had seemingly lost fighting practice). Add to this mix the fact that Shanks probably knows about One Piece's contents and location and he is without doubt the PK in all but name right now. He just doesnt want all the hype…
It may be that like Rayleigh, he has (having knowledge of One Piece's contents) come to the conclusion that it isnt worth fighting over. Heck he may even have taken on the task of preventing One Piece and its incendiary truths from ending up in the wrong hands. Shanks may be the only thing standing between the world and complete chaos, especially if someone like BB gets their hands on the knowledge of the Ancient Weapons
Why did Gold Roger disassemble his crew and gave up to WG after going to Raftel and knowing the true history ??!!
Why did Rayleigh gave up being a pirate ??!!
Why did Buggy who doesn't waste a chance to get treasures didn't try to get one piece although he knows what it is ??!!
I think it's because they knew something that will affect the world but they couldn't do it, and that's why Roger made every pirate in the world to start searching for "his treasure" so that the chosen one will find it and will know every thing they knew and will do something they couldn't do.
So Shanks not going after one piece doesn't mean it isn't worth it, it means that there is something even Shanks and the previous PK crew couldn't do it.
and you can see the look in Rayleigh's face when he told Robin that they knew the true history .
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