Acknowledging that some places can only or best be seen by ship and the advantage of seeing multiple places w/o packing/unpacking, nevertheless one could put together many extremely luxe land-based travel itineraries for that kind of money (and less) that allow multiple days per location at the finest hotels, Michelin level dining, private tours, limousine transfers, etc. Plus you never have to worry about missing "all aboard" deadlines after just mere hours per location and most of it can be cancelled within days of the travel date without penalty.
Perhaps there will continue to be enough cruise addicts and/or people for whom money is no object to remain successful, but IMHO the cruise business structure, especially at the high end, is dated and out of touch with online generations currently reaching and becoming an ever increasing percentage of the retirement travel market.