The main quest isn't a few hundred hours, it's 30-40. A player who focuses on the main quest first will still have a whole lot of game left after the main quest is over.
We're on a different page. I'm not sure how many folks play only the main quest straight through; my hunch is a low number. For most I'd wager that they put in double or triple. I usually go through many side quests and running up the guild as I go. The "game" is more than just the main quest.
I can see it now, our hero has defeated Alduin and decides to join the Companions ...
Guildmaster: So you want some work, huh? Well you are in luck. Old Lady Dauthmer is having some problems with rats in her basemant.
Dovahkiin: Rats?
Guildmaster: Yes, rats.
Which is why one plays through the guilds as the rise in power, and not run the MQ straight through. Near as I can tell, you're kind of proving my point?
So again, I still the it's a question of scale. I don't know what a 3 hour ending would be like, but it should be at least that.