The President of Carnival said, in no uncertain terms, that he will not consider such a policy change. He said, flat-out, that service quality can only be assured at the level of current expectation by the current system.
Rather, I think the writing is on the wall for US-flagged ships. I don't think that model is viable, given the expectations for service, the ability of non-US-flagged ships to provide it in spades, with lower cost-of-service, assured by voluntary gratuities.
I think the hotel example won't apply, because hotels in the United States have to be staffed by employees covered by US labor laws. Cruise ships do not.