The Princess loyalty model is one I think the rest of Carnival Corp should follow. The program maxes out at 150 sea days or 15 cruises (on the 16th cruise you are elite) and has limited published benefits.
They don't give away much, but do offer some discounts, which of course, results in increases onboard spend.
What they do have, and what is sustainable, are certain additional benefits granted to those with the most sail days on each cruise. That number and cost is the same no matter how many elites are on the cruise. If you don't have well over 1000 sea days, you will have to try harder.
This is not bribery, but well played marketing.