It SELDOM happens that all leads leave the same time. They have individual contracts, and unless there is a problem, it is up to the actor to continue. They have to weigh many things, but usually current offers/opportunities is high on the list.
There is seldom a mass exodus from any cast. I can only think of Spring Awakening in recent memory that did this: Both leads (Michele and Groff) left together and then the majority of the remaining core cast departed together later that year. I'm sure there ARE other examples, but it is NOT the norm.
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.