House was also the smartest option.
Well if you ignore the whole effect of the entire NCR collapsing because of it... And that the Legion will persist...
And millions will die after the NCR collapse...
And the whole Enclave thing.
It might be the smartest option but it comes at the biggest price.
Honestly nothing goes right for this location pretty much period.
How do you figure most of that? The NCR doesn't need the Mojave for anything except the electricity, and even that's a debatable "need". The NCR has survived and expanded rapidly before it ever reached this area.
I haven't seen the Legion ending, maybe it involves the Legion invading and conquering the NCR, but that's completely ridiculous...
For one thing, the supply lines just aren't there. As you can explain to the Legate with high barter, the complex economy of the NCR states wouldn't allow the Legion army to supply itself from conquered towns. The towns rely on each other just to survive (The NCR's "weakness" being their greatest defense). The Legion was able to conquer the East because every village was a self-sufficient community. To do the same in the West, the Legion would have to do more than overwhelm a single Dam out in the desert... It would need to blitzkrieg the entire NCR home area. Which it plainly lacks the power to do.
The Legate also admits that the war *just for the dam* has demanded dangerous amounts of manpower from the East. With Caesar on campaign (and soon dead), the remaining patrols are needed to violently keep the various (again, self-sufficient) tribal communities in line. They don't have reinforcements to overwhelm California with, and if they try they'll collapse.
Besides all that... A free or House-run New Vegas isn't even bad news for the NCR. It's the *status quo*. House explains that New Vegas gets plenty of electricity despite shipping 90% or more off to the West. He has no reason not to continue trading electricity and entertainment for military protection. Even after the Legion falls apart and his securitrons are upgraded, he still needs the NCR... Because they have the people and infrastructure to fulfill his stated dreams of rebooting the space program.
A win for House is a win for the NCR, they just don't get to be in charge. The wild card option... Is trickier, depends on Courier's decisions and luck.
Edit: Though I, uh, haven't actually seen the House ending yet. So maybe the ending spells things out and I look stupid for theorycrafting against canon. I don't mind spoilers though, obviously.