In defence of the game getting boring after multiple runs, this is an incomplete act 1. Even if they expand the party to 6, the full game is going to have way more content. Hell, even act one of the full game is gonna have more content. Not just moonrise tower, but the shadow-cursed region which presumably will be about as big as the underdark. Plus potentially more companions as well, so there's still gonna be a huge amount of content to potentially see even with a party of 6.
