So you don't actually have an argument in favor of forced ending. Got it.
In fact, you appear to be (unintentionally) making several arguments in favor of
not having a forced ending, while you're blatantly misconstruing Rushy's arguments.
The fact that you're talking about Skyrim at all goes straight to the heart of my problem. Fallout is not TES. They both have large, fanciful open worlds as settings and RPG mechanics that encourage extensive character-building, but their themes, stories, and roles of the player characters are very different. For example, your comparison between the main stories of NV and Skyrim is unfair. The bulk of NV's main story is you picking a side and then making preparations for a key battle that you know will happen at some point in the future. There's no hurry. Nobody is pointing to you and saying, "Only you can save the world, hero of destiny!" It just so happens that the battle is about to begin by the time you've completed your preparations. If you feel that there's any kind of urgency hurrying you along the main quest, then you're simply mixing up the gameplay and story. Obviously the battle at the dam won't begin until you get there, because a video game is hardly going to shut its main character out of the climax - but that doesn't mean that your allies are sitting at the dam, going, "Oh, man, where's the Courier? We are so fucked without him!"
Not being a "hero's story" gives the story
more urgency, not less. That's because the pace of the story is dictated by outside forces, not the player character. There's no reason why the primary factions in NV are waiting for potentially several in-game years for you to arrive, but
because they are doing so regardless, it inadvertently turns the game into a "hero's story", which you claim it is not. The fact that nobody in the game speaks in urgency is nothing but sweeping the issues under the rug. Their actions speak for themselves, or in this case, their inaction.
You say it's "simply mixing up the gameplay and story", which is exactly what a good developer should do. Failing to meld the gameplay and the story into an internally consistent world is a failure in game design. There's even a term for it, it's called "ludonarrative dissonance". Allowing post-game play would solve this issue, because with it the player is allowed to pick their own pacing. You can either respond to the urgency necessitated by the factions within the game's story and do optional content after the main quest is over, or you can choose to stick fingers in your ears and go la-la-la-la while not giving a fuck about it and make them wait for you forever. Both of these options are fine, because they allow the player to choose how invested they are in the story; with a forced ending, the game is essentially telling you to not be invested at all, unless you're willing to sacrifice a massive amount of optional content.
Furthermore, while the central conflict of any TES game is essentially a hero who has to save the world from a supernatural threat, Fallout's general scope is much more political and societal in nature. In a TES game, little that you do really matters to society and its population (with perhaps a few exceptions, like the civil war) beyond whether or not you saved the world. But Fallout is about the choices you make and the world that you leave behind. To put it another way, the real star of the series isn't the player character, it's the setting itself. Even F3, to its credit, kind of got this, at least with quests like the one in Megaton and the one with Harold. To just turn this into TES with guns, a shooter where you skip across the wasteland and blow shit up and nobody really cares what you leave in your wake because it's all about you - well, that's not really Fallout. Not to me, anyway.
If Fallout were to not be a hero's story (although it is, as I've just now demonstrated), that would only give more reason for post-game play to be allowed, because a forced ending would conclude the player character's story, not the entire game world's story. New Vegas keeps existing and changing after the events of the main story, and the fact that you're not allowed to see any of it only reinforces the game as the "hero's story", because it's about the world
you leave behind, not the world itself. Once again, allowing post-game play would only give credence to what you want Fallout to be, not the other way around.