People keep talking about the MQ Ending as if it deletes your saves, or erases the game from your Steam account. The only difference between "keep playing from a save just before the Grand Finale" and "keep playing after the Grand Finale" is if they actually altered the world with the effects of said Grand Finale. So all the stuff you just laid out - "tons of lore, NPCs, quests...beautiful land"? None of that is erased and destroyed by the MQ daring to End the game. None of it.
The question of whether they would or could make a bunch of post-Finale changes to the world is a separate issue. For one, it depends on how large an event the Finale is, and whether they'd bother to spend the dev time on it. They certainly didn't in Oblivion & Skyrim, and those weren't really "huge change" endings like, say, New Vegas's was.
edit: and anyone trying to play the "but, but, but.... IMMERSHUNS!" card.... I think your immersion would be more screwed up by wandering around in a post-Finale world that doesn't show any effects of it, than by going back to a pre-Finale save and going exploring. But maybe that's just me.