i think all of those folks willingly left the show - except, maybe, pa. but he's had trouble with the law since the day he was born.
i never really understood why people would leave successful tv series, even if the show isn't at its best anymore. seems like it'd just be easy money to collect -- you can continue doing movies in your off time!
7 years is a long time and seems to be a magic number for successful sci-fi shows. look at the successful Star Trek shows, Buffy, X-files. In all of those shows the main cast wanted to walk after 7 years. and the X-files is proof that it's not a good idea to keep going with a reduced cast.
