Where does it say the whole NCR is gone overnight in 2077?

The only thing that’s making you think this is a drawing on a blackboard that says shady sands feel in ‘77, and then an arrow pointing later on in time to a mushroom cloud with no date under it.

The NCR was the size of modern day Germany, albeit probably with a smaller population. If Berlin were nuked today, and only Berlin, do you think Germany would just collapse completely within 4 years? They’d just ignore lines of succession and all of their other cities and give up? Why do you think the NCR would just throw in the towel after losing one city.

As for why we don’t see many people besides criminal types, do you expect to see well adjusted people living next to the literal crater that used to be a city? The cast barely covers a few square miles around a recent nuclear disaster, it’s perfectly reasonable that they don’t see a whole lot of government officials and upstanding citizens while trekking through what amounts to a post-post-post apocalypse Chernobyl exclusion zone.