Keeping McGuire and Dunst for 4 & 5 means they can package all 5 movies together several times over for various boxed sets.
Change the actors and you basicly have to reboot the story. Nothing wrong with that, but there'd be no point in ever buying a boxed set.
Change the actors and you basicly have to reboot the story. Nothing wrong with that, but there'd be no point in ever buying a boxed set.