That... was abrupt. Fallout's main quest doesn't just end, it ends the game. But, but, but.... there was so much I hadn't yet explored! Hell, I had only just been able to get the Perk that revealed all the map locations. WTF?
When playing the first couple Myst games, I saved the game right before I completed the last puzzle for this very reason. Then I could go back to the saved version to go explore other things before ending it.