It all comes down to delivering an entertainment product for the guests. The cruise part of delivering entertainment is minimal.
The Cruise Directors have defined ship duties, such as announcements, life boat drills, emergency communications, and a few other "cruise related duties", otherwise their job is to deliver entertainment, activities and resolve some guest issues. These are the same as it would be at a theme park, Vegas resort or other venue.
All venues have issues and limitations of stage size and theater layouts. This job will be to develop the concepts and strategies to deliver current and new entertainment to the guests. The tactics and delivery will be worked out with this person and those who work with and for him. Just like the Captain of the ship; he doesn't drive the ship but he sets the course.
I have no issue with someone who has a multi job/company track record. Unless they have not stayed at each job or company for more than a year. In my career I worked for five different companies in 26 years. In all but one of those moves I was recruited to the new position and did not actively seek it.
Even if someone is with a company for only two or three years they can change the direction and implement new strategies that can then be used for years after they have left.
It is always disconcerting when the "old guard" changes but Carnival is changing and new people and ideas will be required to fulfill that change. Will it be better? We'll have to wait and see.