User:Alex Sora 89 here (theory partially suggested by my girlfriend Faith). Basically, the only reason Mario must get one million coins is in order to pay a ransom for Peach. There's gonna be a Bait and Switch near the endgame, when it turns out the million coins only grants the player access to the final castle, and then the game goes back to the series' roots: an old-school showdown against Bowser. That's because Bowser betrays Mario as soon as the latter gives him the cash. Calling it right now.