but surely they could even release an update, it doesnt have to be a DLC, that could allow us to continue roaming the wastes rather than just being stuck in the loophole of the last mission, with none of the changes specified in the last clip made?
I believe Obsidian decided against doing that for several reasons, one of which I think is that it would have broken the feeling on continuity in the game. Plus at least some people would complain bitterly about how the game doesn't change to reflect the choices they made during the game after the end of the main quest.
If Obsidian had decided to try to implement post main quest gameplay and try to get a feeling on continuity from how you played the game up until the end of the main quest, I think that would have at least doubled the time to make the game and also vastly increased the number of bugs/glitches in the game due to complexity of having to deal with the large number of variables they would have to take account of (there are Four main factional endings and an additional possible Twenty Three optional ending segments, all of which have anywhere between Four and Thirteen variations to them).