They couldn't have been more stupid with the choice not to allow post-end gameplay. The developers of Fallout 3 plain stated that one of the things they learned with Fallout 3's release is that they would never make another game with a definitive ending. Can you people also explain to me why the post-end gameplay mod is one of the most widely used mods ever developed. It's even gone so far as people finding out ways to mod Xbox saves to allow post-end gameplay.
Because there is more than just
one choice in Fallout: New Vegas, it's not like the FEV ending where you can basically make it completely redundant and only have to produce one set of quests and stuff. If the Legion win then there is nothing left to do, if the NCR win then there is nothing left to do, if House wins then the NCR leave and take their quests with them. It would require far too much work and development time.
If you just want to go back to the Mojave with no changes then just load up a previous save, wasn't hard was it? Not like that requires a sperate piece of DLC just so you can wear Lanius's Armour.