So in discussion regarding whether or not the game should continue after the main quest is completed, most of the argument revolves around the argument "It's lame to have the main quest end, then be able to continue on as if nothing happened. I want to see consequences for my actions, see the results of 'The Big Finale.' Having the game end and showing me ending slides achieves that."
I think everyone can agree that we want to see consequences for our actions. My problem with this argument is that it's based on the idea that the subject matter of the main quest needs to be something epic, something earth shattering; that the protagonist must come in and grandly change the world, be it by crowning a new king, purifying the wasteland, becoming the dragonborn, whatever.
Now obviously whatever they've got planned for Fallout 4 is set in stone, but in terms of future games- what if the stakes aren't quite so macro? Let's assume for a minute that the plot of Fallout 4 is "destroy the Institute" (or something to that nature). What if it was just "find your spouse"? What if the main quest is really only of consequence to the protagonist? You finish it, there's a grand finale, but the consequences only affect the protagonist.
This would completely solve the issue of "play after the main quest is over." Your character would have consequences, but the world itself wouldn't be fundamentally changed, making the lack of response from NPC's believable.