When you look at New Vegas and the insane amount of variables and change that is reflected in the end slides, developing a post-main quest gameworld would take a lot of development and resources. Even with more time, I'm not sure if they could do it justice. If they did, having all of the changes talked about in the end slides actually occur in game, I would be all for it. Unless there were more quests I wouldn't necessarily spend a ton of time playing after, but it'd be really cool to see all of the changes. Alternatively, they easily could have had a popup message about playing after the end explaining that the gameworld will stay the same but you can continue playing. But that would be functionally the same as playing a save before the ending.
So the issue about being able to play after the ending has to do with the story. Not just the main quest, but practically every story in the game. New Vegas managed to account for most of the major choices in the game and explained their consequences. It's a reflection of how much content and how varied the choices really are in the game. Contrast with Fallout 3, where the ending does not really account for the consequences of your choices at all, and instead just tells you what you already know about your character. There isn't a whole lot of choices and consequences that need to be accounted for, and Broken Steel, for good or bad, was a feasible project. Look at Skyrim. No real gravity and no major decisions in the storyline, you defeat Alduin and continue, because there is nothing to really account for in the game world.
So if Bethesda wants to let us continue after the main quest and have a gameworld that's consistent with the ending, it is assuredly at the expense of story, choices, consequences, etc. not just in the main quest, but in the entire game. And frankly, that just [censored] svcks.