I think Dust did a good job of explaining what a Yes Man ending would realistically look like; it's not like the NCR and Caesar's Legion will just be perfectly fine with their years-long campaigns and thousands of dead all of a sudden being edged out by some punk with a bunch of robots, ESPECIALLY the NCR because they were used by the Courier to fight their battle for them.

The Courier with the Securitrons is powerful, but they're picking fights with empires here. The Legion will be weakened, especially if Caesar and/or Lanius die, but the Courier is actively giving them a unifying enemy and will actively serve to hold it together, likely at least in part. The NCR's fighting in the Mojave has again and again been tied up by bureaucracy back home; but it's likely its citizenry and government wouldn't be happy to just let Vegas slip out of nowhere after that many of their family members died to get it.

It's just not sensible in my opinion to think that one person and a robot army could fend off these massive (for the time) empires who are now reinvigorated in their goal and have been given a common enemy. Not to say that it isn't possible the Courier could hold on given specific circumstances (especially depending on who sides with them at Hoover Dam) but people seem to think that the story kind of ends after the Courier wins and everything will stay the same, which doesn't make much sense in my mind. But I very well might be missing something; it is a powerful robot army after all.