ITT: "Oh let's spend all this time coding, writing, voicing, etc. this main quest line that arbitrarily ends the game when completed so certain wannabe hardcoeists and speedrunners can feel good about themselves and tell everyone else how they should feel just peachy about all that work gone into your character is wasted, while everyone else is forced to avoid the main quest like a needle full of AIDS."
Obsidian was going to originally make it so you could play past the ending. This is evidenced by the audio files and Geck lines they left behind.
My guess is that it was going to be very similar to what happened in Old World Blues. A bunch of slides happen telling you the future, yet gamewise it had little effect outside of some dialogues. Example, you might get a slide telling you that the toaster was destroyed by being dumped in the bathtub, yet you were still allowed to continue playing in Big MT, and the toaster would never go missing when you returned to the sink.
... Then again, Old World Blues was meant to be extremely goofy like this.
"This isn't Tes?!" No, this isn't Half Life.