The producers would just TELL them to stay? You do know the actors have free will, right? They aren't endentured servants.
Additionally, they don't usually plan closings months in advance (unless it's a limited run).
Heck, at the end of Drowsy Chaperone's run, Cindy Williams was brought in. She started mid-December, and show was closed at the end of the month. I think she played the role for less then 3 weeks. Bob Saget has recently taken over the role of the Man in the Chair and only played the part for about 2 months.
The producers keep casting and replacing until the show closes.
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.