Wasn't Caesar basically winning before the player arrived though? The NCR barely won the first Hoover dam battle, and they weren't even aware of the fact that Caesar had bases on the other side of the river already, plus a bunch of legion spies sabotaging them. The impression I got was that the NCR was spread too thin and was so incompetent that this crazy fascist roman larper was going to beat them this time if the player character didn't come along and get their shit together. I do agree that the legion isn't going to survive Caesar dying though, Lanius isn't nearly as smart, and when he dies too there would be no one to take over.