If Carnival were to become all-inclusive they would fail. Why? Because they would have to charge fares which would be out of reach to their core customers. Carnival sells value. If they abandoned value they'd lose their base and be bankrupt or up for sale within a couple years.
I agree with LHP that they need to do something to have some sort of reciprocating perks with their other lines, such as RCI and Celebrity do. I have diamond status with RCI and because of that get equal perks on Celebrity, even though I have less than 10 cruises with Celebrity. It would be nice if Carnival Corporation did the same. To me it is a golden marketing opportunity.