The nights vs # of cruises has gone on since DCL introduced the different levels. Personally, I don't think taking one or two long cruises shows loyalty to the cruise line so shouldn't jet pack you to a high level. I started sailing with Disney when they only had 3/4 night cruises. The cruises I took when that was all they had and then 7 nights when that was all they had show more loyalty than someone who has come along recently and taken a 15 night Transatlantic cruise (trust me we've done those too).
To me the top level of a loyalty program should be very difficult to reach. That's the only way to keep the perks of any value to the customer and the program of value to the cruise line. When DCL designed the current program, they were shooting to have a certain percent of cruisers in each category. Unfortunately, I think they miscalculated how quickly people would move up and they now have too large a percentage in the platinum level. (Heck, we just take my parents along occasionally and they are at gold level). So if they were redesigning it, I don't think they would do it so even more people were in the top level -- that would require them to scale back the benefits and make them even more meaningless.
What I'd like to see is maybe 5 levels: Keep the current 3 and don't take any benefits away but add a level that you reach at 25 cruises and another at 50. And I think the benefit that they should add for these levels are different experiences - when you've taken 25 cruises or more the activities become a little stale so it is nice to have something a little different and special to look forward to. Back when they had backstage tours for platinum members we were able to do a real galley tour on one of the transatlantic cruises, it included the storage down on deck A, the elevators up to the restaurants and description of what happens on turnover day in port and how they source their supplies in Europe. One of the best backstage tours I've had anywhere and all it cost DCL was a couple hours of one officer's time, a couple bottles of champagne (there were a lot of people on the tour) and some canapés and we could have even done without the food/drink! I hope when we hit 50 cruises we can at least take a tour of the bridge and being able to hit the Mickey horn button would be a goal worth reaching for.