I'm sorry, but it's in the game.
The Courier's fallibility is hard to argue with when it's based on you know, common sense.
You can say that, fine.
But will you say WHY the Courier is wrong then other than, "Well, it's not absolute 100% fact because it wasn't said by God."
Okay, how exactly will they hold the West with the East?
Judging by how little we know about the situation in the East, I can't say anything that wouldn't be misconstrued as wanton speculation or wishful thinking. From what I gleaned about what little dialogue there is on the subject, it sounds like Caesar has the entire state of Arizona locked down or at least most of it anyway. This is reinforced by the fact that Flagstaff is his capital.
But as I said we actually know very, very little about how secure Caesar's eastern holdings are. I'll admit him wanting to take the dam simply to destroy it or whatever is a waste of manpower, but it's not like he can walk away at this point. If he just turned his back on the dam and allowed the NCR to recuperate/reinforce their position he'd be opening himself up in a huge way from a strategic standpoint.