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Ok what's worse: the strawhats waiting a lifetime for luffy to return to them, or luffy becoming immortal and outliving everyone he cares about?
The first scenario reminds me of that Bojack Horseman quote, "Was it worth it to be happy for a little bit even though it ended up sad? Or would it have been better if the whole thing never happened?" I think this isn't as bad as the immortality scenario because the Straw Hats have each other to lean on, and if they reach the end of their life without Luffy returning, maybe they'll be happy—thinking that Luffy's waiting on the other side for them.
I think immortality is one of the cruelest punishments you could put on someone, especially Luffy, considering his unique relationship with death. It's one of the OP endings I hope Oda doesn't use (through Law's Devil Fruit power).
Grief is something you don't get over. It'll always be there, but you grow to live with it. Knowing Luffy, he'd find new people to love and journey with, but they'd leave him all the same, and loneliness is Luffy's greatest weakness. I don't want to imagine this kind of torture on him.