Sooner or later the cruise line loyalty programs are going to have to tighten up on criteria. They can't keep adding levels.
Royal Caribbean does have a way to reward more spending since they give double points if you book a suite category.
I wonder how much pushback a line would get if they said you had to either book a suite and/or spend a minimum of X dollars per cruise in onboard charges to get X points? I think it would backfire big time if they did what some companies do, such as Hilton Honors where you lose member levels if you don't maintain a certain amount of stays per year.