I think Luffy vs Kaido will be more or less a 1v1. Current Luffy, as shown from his training with Hyougoro can casually launch KKGs in G4. I think he is very close to mastering G4.
By learning control over the flow of haki, he can now conserve haki better and therefore has a higher G4 time limit.
However, unless he learns awakening or something, G4 will still have a time limit. So I do think that the help that Luffy will get in the fight will basically just be the scabbards as well as a few supernova stalling Kaido so Luffy can regain his haki. I also do think that it is still possible for Gecko Moria to come and for us to get Nightmare Luffy 2.0.
But overall, I say that Luffy vs Kaido will mostly be a 1v1 apart from Zoro, law, and maybe Kid, no other character has the ability to hurt kaido. All 3 of those characters will likely be dealing with big mom/ the calamities while the war is happening so I don't think they'll be able to help luffy that much. All the other characters can do to help Luffy is stall kaido for him to regain haki, like in dressrosa.
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Is eternal youth not immortality.
Regardless
we don't know how that ability works exactly. But I think its a lot more complicated that simple eternal youth. Again, Doffy said he could be pirate king if he had this fruit. I don't think eternal youth alone would give the ope ope no mi the epithet of being the "ultimate DF" when sugar's DF and Brooke's DF do similar things.
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also, BM needs to get beat somehow. I think this is just the easiest route for Oda to go while still making her defeat satisfying(as opposed to bringing back 'big meme')