Too many variations to account for.
Each DLC would have to be close to an 1600/2400 MSP expansion.
So four expansions just cause you can't reload a save prior to the final battle?
Sounds very unnecessary to me.
And then you would need different endings of the different DLC's aswell, and then people want an extension of that story too?
Or is it just gonna, err, end like Broken Steel did? "You've saved the D.C.! Now, hero of the wastes, go and collect cameras."
That was rather cheap, I felt no closure what so ever to the story because of that. And then you just roamed around, killing innocent people, writing stuff with corpses on the ground, until you got bored and stopped playing F3.