Yes, that's certainly one reason. Another is the staggering capital investment involved in bringing all the new ships online and refurbishing the rest. You can only "make it up on volume" to a certain extent, then you have to try to figure out how to 1) cut costs, and 2) pick up a few more discretionary bucks from each passenger. That's why you're bound to see more "added cost" features, whether it's separate classes, a la carte dining, or charges to break your neck climbing rock walls.
The real crapshoot will be figuring out at what point the cruise market will get saturated. They've been successful in bringing cruising to a mass market; the question now is how much water the sponge will hold. Guessing wrong will be an expensive mistake.