Look at Mass Effect 2 (I think that was it). It had a definate ending...but because it was open world they chose to put the player back in the world
No, they did that because they knew they were going to keep horking out DLC for the next year. And they knew that forcing people to start a new game in order to play "Oh, hey - you can actually resolve Liara's plotline" would reduce sales.
Of course, the "keep playing" thing made no sense whatsoever, since you just did Big Dramatic Suicide Mission?....... and nobody notices. Not the game universe that still wants you to go rescue a puppy from the nearby tree, not your teammates who swore they were only with you until the final quest and would then leave, etc, etc, etc.
Totally stupid.
Why could New Vegas have a similar system? would make me much happier.
Well, for one thing, New Vegas has a billion times more ways that it could end - between the four main finishes, plus how you were forced to resolve your issues with many side factions by the final MQ bits, plus how lots of quests were invalidated by the way you solved other ones (all those "Whoops, Mission Y Failed" messages you got while doing Mission X), etc, etc, etc. Compared to ME2 having, what, one option at the end? (blow it up or not)
Oh, and then there's the part where the continue option in ME2 was
totally stupid.