It's quite easy, Mihawk's not gonna be Zoro's ultimate ceiling - Zoro will end up reaching the powerlevel of whomever Nika's right wing is supposed to be. It's the same with Sanji. Just like how Luffy will completely eclipse Roger during the final saga, there's no reason to assume his wings will cap to ~PK wings at best.
It's quite simple: Roger, WB, Xebec, Big Mom and Kaido couldn't do jackshit to one of those ancient gods who merely borrowed (!) someone else's body.
Sure, you can come up with some cope like PK Roger >>>>>>> GV Roger but you're just deluding yourself at this point because this Roger was close to 40 and near his prime.
The issue is you cannot adapt to whatever direction the manga will go. At the end, story >>> made-up powerlevels, that includes Roger's "legacy." Because one thing is clear, Roger was the PK but he's nothing against Joyboy, Davy Jones, Imu, Nika, all the other gods. He's just another pawn in this story who's meant to lead the current generation towards victory - Luffy is the protagonist who will claim the ending. I think this much is quite obvious considering what we've seen in the mural?