Carnival's market skews younger than those other lines, and although I was recently on a journeys cruise that had a pretty high median age, I doubt there would be demand for a Carnival Cruise for that length of time.
For example, if I am an early retiree. Even though in theory I have the time to take a 3-month cruise, I have elderly parents and young adult children that I would not want to be halfway around the world from them for that long.