It is very rare that a ship returns late. About the only situation that could occur is if there were a medical emergency and the ship had to alter its course to evacuate someone on the return trip back to its home port. Even then, it wouldn't really affect the departure time.
Enjoy life, what if cruises were like airlines?
You would have to wear seatbelts at dinner.
You would have to put all your luggage in an overhead storage compartment in your cabin.
Instead of dining, you would have to pick up your meal in a bag at the entrance of the dining room.
The only bottles of wine you could order would be rollers, and they would be $5.00 each.