Hmm.. That explanation makes more sense to me than the other one, IMO. If a good chunk of the movie is spent gathering the characters who are left and dealing with motivations/aftermath and what-not before even thinking of engaging, then that might justify it. I was under the impression that most of them were already pretty connected and it was just a matter of figuring out how to resolve the main conflict (that part, I would be surprised if it needs three hours and would be skeptical if it was). Again, I hope it doesn't get too convoluted/bloated, though, with time traveling, fighting Thanos and then fighting him again with a bigger resurrected group. Seems like that would cheapen it. Keep it tight.