I mean, why is it music to your ears to lock away the full ending doing this? If they hadn't done that, you would still have the choice of tackling all the side stuff you already like. That never changed regardless. I just don't get why locking away a full ending after the game is over (on a narrative level), is music to your ears...that they attached that on top of something you were going to do anyways, and enjoy.
I mean, if you already enjoyed all of these things, and get the joy out of completing everything, I don't know why you need a "real reason" to do it. Isn't THE reason, because it's like actually fun to do?
EDIT: actually, your view point is valid. I mean, you are basically saying that, locking away something as pivotal as a full ending, gives you a reason to 100% the game. That is why you love it. I don't know though. I just feel like, the side content/puzzles should stand on their own. They should be something you do, because they are great gameplay sections, and something you enjoy. Tying something as important as the full ending to all of that, just seems I don't know. Never been a big fan of this decision. I guess. Oh well.