The question is two-fold. Firstly, without the Courier's intervention, who wins The Second Battle of Hoover Dam? Secondly, who wins the war? The first one is obvious. The Legion has countless operations in the West that will utterly cripple the NCR's operation in the Mojave, while the NCR is stated to be stuck on the Western side of the river. The Legion will win Hoover Dam. But Hoover Dam isn't the whole picture. Hoover Dam is the foothold into the West the Legion needs to wage a proper war against the NCR. Once they punch through the Dam, the Legion's gonna make a beeline for the Long 15 and start spreading into NCR territory. From here, the victor is obvious. The NCR. The NCR's biggest disadvantage against the Legion is that they're spread far too thin. They don't have nearly the same infrastructure in the Mojave as they do in California. Once the Legion gets into California, though, the kid gloves are off. It's implied in a few spots that the NCR is starting to get back to (or at least significantly close to) Pre-War levels of civilization. Their infrastructure is vastly superior in the far West. The further West the Legion gets, the more their strengths wane and the NCR's weaknesses subside. The Legion will get pushed out of California if they ever get close. The NCR might not be able to reclaim the Mojave or the Dam, but the Legion can't take California.