There's no logic behind not allowing post-game play unless your character dies at the end.
Spoiler And your character doesn't die.
Sure there is. Allow me to explain:
In Fallout 2, the "good" ending for Broken Hills involved the town dispersing because the uranium it sold to keep its economy going ran out.
Fallout 2 allowed you to play after the end, and consequently,
to murder every man, woman, child, mutant, and ghoul (and giant spore plant and intelligent radscorpion) in the town.
This option effectively renders that particular ending segment utterly meaningless. If you can go and kill everyone, the town obviously wouldn't disperse because the uranium ran out.
Beyond that, there's also another issue. That is, changes. The changes in the ending are potentially tremendous. NCR taking the strip would need a
lot of new content to properly explore. New dialog, new scripts, new quests... If you say this isn't necessary, what exactly is the point of playing on after the end at all, if there's no new content to explore? If there's nothing new from playing after the end, there's no reason to play after the end, rather than simply reloading the save you make before the point of no return.
They made a huge epic and complete game and that's great, but I'd still like to see a DLC that let's you go back and see the *ahem* fallout of your actions and decisions. In FO3 I thought it was hugely rewarding to go back out and interact with the world I helped shape in Broken Steel. Even if they said they weren't going to do that, it's not like it would be the first time a company lied about a game pre-release. At least in this case it would be a lie to cover up a pleasant surprise.
I don't want a DLC exploring the endgame, because a DLC is simply too small to properly do it justice. We need a full on expansion at a minimum here. Possibly even a sequel. Remember, we had to wait until Fallout 2 to actually see the long reaching results of helping Shady Sands back in FO1, even though we got an ending slide talking about how Tandi formed the "New California Republic."