@Sea:
It's entirely possible to let Kuma inform some character other than Rayleigh. He could create a new character for that purpose. Just have to change some details.
The frame still doesn't change.
And if you change every detail then suddenly you have another frame.
You'd need Kuma to inform a character who was at the scene, was relevant, had enough skill to hold his own and survive after the message, and had enough motivation to actually take that message and do something about it.
You could write WB out as we knew him, or give him a different, build, appearance personality, DF, whatever. The only reason you cannot change his name is because we knew he existed in Alabasta. Sure the frame was there, but you can change a thousand details all the time.
I'm sure we all agree that WB worked wonderfully though, down to the last detail, but so did Rayleigh. They both work. They are both relevant characters that had different roles, but I believe them both to be necessary.
You could say that with WB and Ace went the central conflict that would be building up up to the end of the first half of the story. But Rayleigh's role was to enable the start of the Second.
The crew not being able to proceed beyond a certain point remained just as big a problem as Ace's journey and imminent demise. If you wanted, you could say that another conflict had also began to be foreshadowed since Alabasta, with Crocodile´s first remarks on Luffy's weakness, and from every battle since. The truth is that it was becoming abundantly clear that Luffy and his crew were hitting a wall. And while Ace´s conflict drove Luffy's actions, behind it all, and the final lesson that Luffy learned when Ace finally died, was that he was too weak.
The story skipped two years ahead because the conflict was not entertaining enough to show, but it was still there. And the character or Rayleigh was instrumental in solving that conflict.
You can call bullshit all you want, but Rayleigh was important to this particular hurdle, and it was a huge one. The biggest one really. It just wasn't solved in a week of story-time, so it wasn´t shown.