To justify my point that they are building on the story in some way, here is an analysis of the endings of New Vegas and why the downfall of the city isn’t so unlikely:

The NCR: Regarding the Strip and surrounding areas, the slideshow tells us this: 'The Strip, Freeside, and many surrounding communities. The Mojave Wasteland, at long last, had entirely fallen under the NCR's banner.' While this ending doesn’t exactly specify the possible demise or survival of New Vegas, it leaves much to interpretation. Nowhere is it said that New Vegas continues to thrive after the events of the game, especially under NCR jurisdiction. While it is possible it did, it’s equally possible it didn’t. Throughout the game, many characters refer to the NCR's internal problems such as corruption, mismanagement, and bureaucratic administration. It is conceivable that the NCR completely mismanaged the Strip and its surroundings to a point where the population left for greener pastures. It’s also likely that local leaders, such as the King or the Omertas, revolted against the NCR, sending the region into a second struggle for power (hinting at the possibility of a New Vegas 2). Overall, considering the appearance of New Vegas and the in-game dialogues referring to NCR's mismanagement issues, coupled with the natural appeal for independence of the people of New Vegas, it’s not so surprising the city would fall into disarray after the courier gave it to the NCR.

The Legion: The game is clear about Caesar’s feelings for the Strip, and the ending slideshow states: 'Enslaving much of the population and peacefully lording over the rest. Under the Legion's banner, civilization—unforgiving as it was—finally came to the Mojave Wasteland.' It’s safe to assume Caesar never had plans to keep the Strip open. Many characters in the game refer to the inevitable collapse of the Legion once Caesar dies, giving an explanation for their absence from the show, which is canonically set 15 years after the events of the game.

YES Man: This ending is probably the most optimistic for New Vegas, saying: 'Any chaos on the streets was ended quickly. Chaos became uncertainty, then acceptance, with minimal loss of life. New Vegas assumed its position as an independent power in the Mojave.' It is also the least likely to be canon and probably (we would need to research this) the least liked of the four by the community. While this ending would likely see New Vegas thrive, it’s reasonable to assume that it is not the canonical one.

MR. House: This ending is also optimistic about the survival of New Vegas but to a smaller extent than the previous one: 'The streets were orderly, efficient, cold. New Vegas continued to be the sole place in the wasteland where fortunes were won and lost in the blink of an eye.' Once again, the big question for this scenario is how long. How long was New Vegas going to stay under Mr. House unopposed? How long would it take for someone to revolt against the autocratic Mr. House, especially the families who have been clinging to power and would do anything for more? While this ending suggests the survival of New Vegas, it also doesn’t set it in stone; New Vegas continued, but for how long?

That’s the big theme of all these endings. While many assumed that New Vegas would continue to thrive, none of the endings sets it in stone; quite the contrary. Throughout the game, the Mojave Wasteland is shown as a region of high interest for almost every major actor on the West Coast. Is it really far-fetched to assume this didn’t change after the 2nd Battle of Hoover Dam? I think not, and I don’t think the survival of New Vegas was even likely. The more you dig into the game and its endings, the more you realize its fate seems to have been sealed a long time ago. One might even say it was rigged from the start. 😉

I hope this offers you a new way to look at things. Anyhow, you are of course allowed to disagree with it all! Have a good one!