I'm reasonably sure that Obsidian decided against post ending gameplay for several reasons one of which I think is that with the timescale they had to work with they knew they could not take into account the characters actions throughout the game and thus couldn't create a 'seamless' post ending gameworld. Also the game would have to keep track of the various ending segments you can possibly get, of which there are a total of 27 different segments you can unlock, although one of them is negated by one of the others, so there are a total of 26 ending segments you can see at the end of the game. On top of this each of those 27 segments has anywhere between 4 and 13 variations to them. Trying to keep track of all that would increase the games programming complexity by at least an order of magnitude and on top of that the more complex a program is the greater the chance of more bugs and glitches.
@WestoftheRockies. The Vanilla version of Fallout 3 had a definate ending so your example is incorrect citing it as a free roam style game. Mass Effect 2 while it may let you continue playing after the end of the game, it's post ending gameplay leaves a LOT to be desired.
I do agree with you. Im not that bothered about the game ending, it gives you plenty warning this time which is good.
I was just thinking maybe they could let you still explore the mojave searching for side quests without any of the changes after the end. Sounds pretty silly to me but maybe it would stop some of the people complaining. Having the choice there wouldn't do any harm?