Broken Steel didn't fix Fallout 3. It literally Broke Fallout 3's story.
To have free roaming after the ending would mean you have to remove all the possible endings, which would make the entire game pointless.
Right. It is a good idea to add freeplay after main quest but in FO3 it is poorly done. For free play it is better to let the main quest unfinished than to play broken steel.
In FONV it is difficult to add freeplay since the world is small and there would be little left to do, even less than in FO3 and it would be very difficult to add freeplay for each ending (cesar dead, Kimball dead, legionor NCR ruling, Kings, great khans and followers wiped, powdergangers and fiends ruling ot not...) And honestly I am glad to end the game and start a new one after some time. The end is boring when the courrier becomes too strong