One of the posts above infers that they were able to leave at the end of Voyager. Don't know if that's true or not. If it is, then why not leave? They had 12 months knowing that the seventh season of Voyager would be the last.
If the statement was false, then there are ways of maneuvering yourself out. I'll use the Straczynski example again. Refuse to take network direction. They either replace you or let you get away with it. If they replace you, you're freed up for other work.
That's why JMS is stuck doing comic books and can't get another B5 project off the ground. The man only had one creative project in his head for what is now close to two decades.
To answer your first question, I'll ask you this: Why do you work?