As income disparity increases, there will be a smaller pool of people whose demand for cruise will rise faster than their rapidly increasing incomes (as is the nature of luxury products). The margins available from each of these customers will grow over time (hence all the new ships chasing their £/$), but the number of customers will shrink over time and of course they can only cruise <52 weeks of the year!

There is a genuine cliff-edge somewhere in the future, but it's a long way beyond the accounting life of the ships currently being built, especially given the emergence of new markets in Asia.

If you get over saturation of luxury ships it would be indeed bad for that market.
To attraction the clients prices would have to be lowered, as a result corners would have to be cut to protect the bottom line, and as a result clients would not be receiving the full luxury experience they would come to expect.