You know what'd be a good thing? Alternate endings.
Let's say a player does the following:
He blew up the robot army.
He deactivated Mr. House.
He destroyed Yes Man.
He did not gain a single ally for NCR and failed every main faction quest that was given to him.
The final battle would be unwinnable. Well, an illusion of 'chance'. Player would have a 100 Speech check to convince General Oliver that NCR should pull back. Succeed, then you'd have to cover the NCR retreat...
Fail,and you'll fight in an unwinnable battle at Hoover Dam.
Player is basically given the objective of 'Hold the Line' in Hoover Dam finale but there was no end to it. You just kept killing waves and waves of Legion till you ran out of health OR desert your position and everything fades to black.
Either the slides tells that the Courier barely made it out alive from the Mojave or...if he stays and dies....
The player awakes, crucified in the midst of hundreds of Caesar's Legion. Now THAT would be the ultimate epic losing end than a game over screen or "You won a marginal victory" slides...
Reverse that with Caesar's Legion being totally unable to fight the NCR...you end up being captured then executed by Rangers Firing Squad along with the rest of the Legion's main commanders.
But then again, the Legion faction writing sucks dick. And they literally sucked each other dicks as well.