Essentially
I think the issue is, he's been very coy about how the final structure with the war is going to go, but if you put the pieces together, it becomes clear Luffy can't just magically join this war after Wano.
One Piece has been about the journey for 25 years, to have the crew go back to the Final War front (assuming it's New Marineford/Mariejois) without actually finishing their journey would be really odd and frankly not good. It's important we reach Lodestar I think, then the crew can head backwards towards Laugh Tale (wherever it is) and onto the Final War.
I'm sure it'll be far more complicated than this, but Oda definitely just doesn't want people figuring out the structure of his endgame.