Once a movie leaves, is it gone forever or will ever return?
What do you mean?
Some movies do come back, but not soon
Leaves where? Cinemas? Sometimes they come back for a while, sometimes not. Usually they then go to DVD or are available to stream.
Do you mean theaters? Once a movie leaves theaters it doesn't return unless the studio decides to re release for it's 20th, 30th, 40th, etc anniversary.
might return, not always