I've made given my thoughts on this before, but here goes again.
IF done right, a sequel to a film can be more successful and rewarding than the original. Taking a world that is known already, characters that have deeply built histories, personalities, and lives, and creating a new story in that world can be a major benefit. In Toy Story, you spend the 1st movie basically creating the world, creating the characters, and getting people to "believe" this entire world, while telling a story. In the superior TS2 and TS3, this is all done already, you dont have to spend the time convincing the audience to like the characters, root for them, all you have to do is tell a good story. The creators of these properties have a great affinity to these characters, and it is not a surprise if they feel they want to revisit them to tell more stories. It's kept the comic book business going for how many years?
IF the movie is done strictly to cash in on a previous franchise, using the trust built by the first movie only to guarantee a certain number of ticket sales/merch <cough Planes cough>, then it's a waste and the company should instead focus on new IP.
