I saw on a TV special about cruise ships that the average lifespan of a cruise ship is expected to be 30 years of regular service. With all the investment that goes into building these things, I'd doubt they'd want them replaceable in only 20 years. I sincerely doubt the Magic and Wonder are going to be sold or retired
any time soon.
As for new ships, it'd be silly to think that they haven't already begun the planning stages for future ships. You can plan a ship without purchasing one, I'm sure. A previous poster mentioned that the original plan was for 10 ships. Frankly, if Disney wants to keep their repeat cruisers, they need to begin offering more--which I see plenty of threads on these boards calling out for. With more ships, I'm sure they could offer more options. As for troubles filling them--I certainly do not notice this problem. The discounts generally are only found in the off-season, which all cruise lines offer. Since Disney is targeted primarily at families, of course the off-season itineraries would have a hard time being sold due to the school year.
My issue is that the prices are just astronomical. Yes, the service, amenities, and atmosphere are high-quality... but for the same basic itineraries, it's crazy how much they're upcharging for the new ships. I just priced a June Fantasy cruise for next year and it's more money for an inside than what I paid for an outside Alaskan trip on the Wonder for this year. However, I also checked RCCL for the same time for the same itinerary, and the prices aren't much different at all--for their newer ships. Of course Disney would probably want to continue that demand for "new" by introducing another ship in the next several years.
This article talks about RCCL's upcoming new ships--smaller than the Oasis class--which took 3 years to design. They'll be coming out in the next 2 years. Their latest ship, Allure, debuted in 2010. That's a new ship, at their current rate, of every 4-5 years. The Dream debuted in 2011. If
DCL follows the same model, it wouldn't be surprising to see another new ship in 2016.
I'm sure the big fleets can fill their ships because of the competitive pricing and the variety of itineraries. If DCL has any problem filling their ships, it's because of they lack both of those. Their "new" itineraries tend to be limited and only offered at off-season times. That excludes many families, and those of us with limited time off. I say that more ships for DCL is a good idea.