The "concern" that service would suffer if the gratuities were not adjustable...is the same "concern" that was expressed when Carnival started automatically adding it to the Sail and Sign card.
We had done about 10 cruises before they were automatically added (when you handed everyone cash) and about 30 since the change and we have seen no decrease in service.
If these were handled like service surcharges (which is what they truly are), I do not believe the service would suffer because these individuals are professionals who take pride in their work.
The very FEW who sluff off will do so no matter how they are paid.
Unfortunately, (like the 50% we just experienced on the Destiny)...you have a culture in America that does not tip. That means many cabin stewards and wait staff worked for free last week.
Perhaps if Carnival made the service surcharge nonrefundable on groups of more than 25 (like restaurants do for groups of 10 or more) ... then the majority of the workers would be protected.