Also Rayleigh was still using the coat 2 years after Roger's death:
http://www.onemanga.com/One_Piece/565.5/22/
Does Oda seem like he does simple things? Well sometimes he does, but most of the times the plot is programmed from way back. Like who'd know that Luffy would meet Jinbei and that he wanted to thank him because of Arlong?
Or that they would meet Brooke after meeting Laboon?
Anyway, people are saying that they don't know why WB didn't want to become the king. Roger was called the king, he never aimed to that title.
WB is the one closest to become the PK but he hasn't. Why? Because he sought after something else. We know that he loves his sons more than anything and he's done great things for them.
What if what he's looking for is to protect young men/women because he wasn't able to protect his own son sometime in the past? That could explain his talk with Squardo about "Do you know what a parent feels when a son dies in front of him".
WB is trying to secure the new generation. That's what I think he sought, the protect them.
Same with Roger. We don't know what OP is but we know that Shanks was on the boat, so was Buggy. We never actually saw any of those being all over OP so I'm thinking that it's not a physical treasure (like many think).