If
DCL stops going to Nassau on 3 and 4 night cruises, where else could they go? They clearly want to stop at
Castaway Cay,
Lookout Cay, or both. Many years ago some 4 night cruises stopped at Freeport, but that was even less popular than Nassau. There aren’t a lot of other ports in the immediate area that can take big ships and are significantly different/safer from Nassau. So eliminating Nassau would probably mean adding a sea day.
People who don’t want to debark in Nassau can treat the day as a sea day anyhow. The main difference from a “true” sea day is that shops are closed, but if you really need to buy something during the day, you can buy it on shore.