User blog comment:FairyRaven/Fairy Tail Chapter 425: Sabertooth X792, Review and Commentary/@comment-3987368-20150323105114/@comment-3987368-20150323113626

It's just something that predicts how they'll be in the future, and relating them to the current (deceased or otherwise) generation of senior pirates. Particularly, Luffy being the future Pirate King (Gol D. Roger), Zoro as first mate of the king (Rayleigh), Kidd as the psycho-but-strong rival (Shiki) and Law as chief rival and powerful equal (Whitebeard). Thus, a lot say that this is why these four won't die in the series. Not saying it's gonna be the case, but I can understand why they feel this way. Given how other manga dealt with generations preceding the storyline mains (e.g Naruto), we can see why.

If you read Toriko's manga, I think you ought to catch up (if you aren't already) and start talking about it weekly.