The problem is that, to actually see the changes, you'd need to have four (five) radically different settings. An ending where Caesar won at the dam means that nearly every settlement has to be reworked. Ditto NCR, House, or Yes Man. All those NCR quests you never got around to doing? Those are closed off since you just evicted the NCR from the Mojave. Etc etc etc.
We're talking full 30-40$ expansion territory to properly do the concept justice here, minimum. Not a cheap 10$ DLC.
There's also another problem with playing after the end. Namely, why? NV's unguided exploration is not particularly good. The meat and the interest in the game comes from its quests. It's not like FO3 where every location is a dungeon filled with raiders/muties/Enclave guys to shoot. We can see what happens when games that are quest driven have freeplay after the end. Fallout 2 had this, and it rapidly became pointless. Mass Effect 2 has this, and it also rapidly becomes pointless (except for a handful of DLC; which you then complete and it becomes pointless then.)
Very true, there are 26 of a possible 27 ending segments which you can activate with each playthrough and of those segments each one has anywhere between four and thirteen variations. Which means that for Obsidian to create a feasably playable post ending gameworld consistent with how you played through the game would probably take just as long if not longer than it took them to create New Vegas in the first place. Plus another point to consider is that for them to ensure that the post ending gameworld does actually take into account what you did during the main quest would increase the complexity of the programming expotentially which would also increase the number of bugs/glitches by several orders of magnatude, as the more complex a programme is the greater the number of bugs/code conflicts/glitches/problems there are.
With regards to the last point in that NV is more quest based than exploration/dungeon crawling based again true enough, although I think that possibly part of what those crying for post ending gameplay are wanting is purely the ability to continue playing after they've got the main quest out of the way (disregarding that this removes the majority of the quest based content from the game) so they can then play the DLC's without being 'forced' to create a new character to play the DLC with.
@Lyon's Pride. To be 'pedantic' at least to start with in this response. I take it that the vast majority of games are 'completely stupid' because the vast majority of games of most genres end once you finish the games main quest, also what I was initially trying to say with my prior post in this thread that it
IS technically a consequence of you the player not creating a save for the purposes of DLC play. Also if you look at older games with expansions which have endings (which is most of them, at least in my experience) you have to 'restart' them to play the expansion so the fact you 'have' to restart New Vegas to play the DLC if you do not have a save at a point you can still access the entire gamemap is not 'wierd'.