The airlines have been canceling low-booked flights for a few years now. What would prevent PRINCESS CRUISES from doing the same thing??
Airlines cancel flights but still get the people who were on them to their destinations. They simply can't afford to waste fuel with two airplanes when all the people from both flights can fit on one airplane. It's called resource and yield management.
Cruise lines will drop prices to spur bookings for a cruise if needed, and I cannot see Princess or any other line canceling a cruise and leaving the ship tied up at the pier for a week. That costs them more than sailing with a half-empty ship. A cruise line profit margin isn't anywhere near as small as an airline's.
I have seen an 11-day cruise canceled and replaced with two shorter cruises - a 7 night and a 4 night. This proved to be more successful and the ship was sold out for both of the new cruises. History for this - the Song Of America in December 1993. We booked the 4-day which they sold for $200 per person outside cabin, with $100 air from ATL to SJU. It was an amazing deal. The cruise was from San Juan to Miami with one stop at St. Thomas.