No kidding! I can't believe they made the same mistake FO3 did.
Good thing it wasn't a mistake then.
Marketing ploy, I see DLC which unlocks the game after the MQ in the future.
Impossible.
Unless they release a series of DLC:
"DLC #1 - The story continues after an NCR finish"
"DLC #2 - The story continues after a Caesar's Legion finish"
etc,
etc,
etc.
The way NV ends makes it impossible to continue without altering the world in big ways. Much moreso than Fallout 3's "Oh, hey - clean water!".
Especially with the way that, in doing the finish, you commit to one of the main questlines (locking out all the others), and so many quests are mutually exclusive (Oh, you finished Quest X this way? You can't do Quests Y or Z.)
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Thing I hated to do in my first playthrough - blowing up
near the end for my MQ faction.