Having game play after the main quest ends will remove the endings were we learn what happens to everyone. If the Legion take over, everything will be under legion control and same with NCR. That would mean a lot of work remaking everything. Same if you and Yes Man take over or Mr.House. It would be like making a whole new game. Fallout should not be a sandbox. It can't be done without getting ride of the mutiple endings that are awesome.
Also Again whats so hard about loading a save before the end? Or paying attention to "the game will end if you keep going?"
Yup, not to forget a crapload of new dialogues because you don,t want NPCs to still have generic information to say about each faction. Craploads of dialogue means more voice acting. And that costs a big chew in a budget. A budget a DLC couldn't afford.
Doing so would require, at most, an expansion pack. And a big one too.
I just completed the game for the second time, with a whole different playstyle. The endings are simply awesome. More so than Fallout 1, I'll admit..since you also learn what happens to your companions.
After having made the independant charismatic bastard and the pro-NCR old cowboy, time to make some melee-weapon happy pro Ceasar's Legion and see what happens in the Mojave. :foodndrink: