Let me start this post by saying I have never been on a Disney cruise.
Why would anyone without kids/grandkids go on a Disney cruise? The way I see it, you are paying X amount of money for Y amount of product. Product includes: food, drink, entertainment, service, ambiance, and other things. On a Disney cruise, a significant percentage of the product is directed at making it fun for the kids and at things that are more likely enjoyed by kids than adults. I would think that it would make more sense for 2 adults, travelling without kids, to spend their time on a ship that is less focused on the kid experience.