One likes to think one grows as a writer as one ages, else all you get is an 'old' young writer. Beyond that is the changing landscape of the universe and the stories I choose to tell.
The group started getting bigger and bigger, so Al started replacing Brian on the road, and then finally there was a big flare-up with Dave Marks and he left the group.
My guess is that if they now choose to change of director for every other film, it's just because you can't really change the formula, you can merely try to film it your way.