Kimball and Oliver are indeed screwed if the Mojave Expedition fails....but Caesar and Lanius would survive defeat just fine (unless the Courier kills them). Caesar would present it as a temporary setback, and/or a test by the God Mars to prove their worthiness. Lanius, of course, would kill anyone who backtalked him. Anyhow, just because Kimball and Oliver are not very bright doesn't mean they aren't right in this instance....as they say a broken clock is still correct twice a day. Control of Hoover Dam and access to the Colorado is a worthy strategic goal that will be of immense benefit to the NCR in the short and long term. And Clark County, NV is a hotbed of anarchy right on thier border, threatened by the Legion, who Kimball correctly identifies as dangerous to the NCR. Having no knowledge of House's plans...no one does but House...it's clear the good residents of Clark County will be overrun if they don't do something. The only defense they had was the Desert Rangers, who were worn down by the Legion to the point they had to petition to join the NCR to get the support then needed to keep up the struggle.
And Hanlon...his Smedley Butler act doesn't entirely hide the fact that he despises the residents of the Mojave as ingrates and doesn't really care what will happen to them if the NCR pulls out....and the Legion steps in to fill the vacuum (don't forget he has no idea what House is up to..he thinks House is just another shortsighted ingrate). His point of view is only vindicated if House wins....as he can safely play Smedley Butler in the NCR Senate, knowing there is a strong buffer state between the Legion and the NCR. If the expected happens and the Legion wins, he ends up dead in most cases and in the rest will be contending with the Legion as the AO moves to the NCR's border regions.
Though I hate to admit it, the Legion actually has a sound economy...it's money is worth four times the NCR's paper money. It's true that they need conquests to bring in slaves, but Caesar isn't attacking the NCR because he has to in order to keep his economy afloat...he even tells you if Kimball had staged a coup and made himself Dictator instead of entering politics he wouldn't be attacking the NCR at all....he's doing it for his own personal glory and to transition his Regime from a nomadic Tribe to a Nation-State (albeit a Totalitarian Dictatorship with him as Maximum Leader). Like most Totalitarian Regimes, the Legion depends on a Cult of Personality surrounding the Leader. The best way to remove the Legion as a threat is to kill both Caesar and Lanius and deflate the cult of Personality once and for all. Caesar has to go as he is the most effective leader they have and will never waver from reaching what he sees as his Destiny, and Lanius too as he will use his own strength and Caesar's legacy to keep the Legion going in the short term....consensus seemed to be a year or two....meaning a lot more people will die before the wheels finally come off the Legion's cart. With the pair of them dead, there is no clear successor....which means the knives will come out as ambitious Lieutenants start dreaming of being Caesar themselves. The Legion will tear itself apart, all the NCR or House has to do is stand back and break out the popcorn. Once the smoke clears, they can start making deals to pit the little Caesars against each other and manage them all as a annoyance rather than a existential threat.