Shouldn't Nami be able to visit the entire world's ocean by being with the Strawhats? Not sure if Usopp can return to Kaya after being with Luffy and being the one who burned down the World Government flag, even if he did it masked. I don't see the crew disbanding without Luffy ordering them to, as it's basically the bonds that drew them in akd kept them together no matter what happened. The only reason why Rogers, as far as I know, disbanded his crew was because he was dying, and there was no reason to stick together. I could see Strawhats sticking together for a long time.
But I really don't see Luffy dying, not anything short of a horrible disease because of his durability. The injuries he's gotten and the power he's used will likely affect him, but I don't think it'll shorten his life so much that he'll die within the next ten or twenty years. Though, there could be a timeskip where Luffy's dead, but his legacy still lives on and he's honored deeply by numerous people.