It's just lazy to criticize sequels and paint them all with a brush. There are great follow ups, and Hangover 2's that just recap the same movie in a different location with a couple new characters.I agree. Sequels get a bad reputation. Not all of them fizzle out. Toy Story 3 and Kung Fu Panda 2 are great sequels! So are The Dark Knight, The Empire Strikes Back, Godfather 2, Aliens, Indiana Jones & the Last Crusade, X-Men 2, Hellboy 2, The Great Muppet Caper, Die Hard 2, Evil Dead 2, Blade 2, Spider-man 2, Lord of the Rings and Harry Potter sequels and any number of Bond films. Not to mention that the next Muppet movie isn't technically a sequel.
Dark Knight clobbered Batman Begins in the box office. It's a far superior movie, sure... but for box office cash value, it completely eclipsed the first film. If only they made a sequel to Superman Returns... I'm sure we'd have a strong Superman franchise by now.
As for family films, sequels actually do quite well, otherwise we wouldn't see 3 and 4 of them. Shrek had 4 films! If the second one flopped, that would be it. Most of the big family films have too many sequels, if anything.
And there's something else a sequel brings when they didn't realize how much of a success the first film was. More merchandising. Fox didn't have that much faith in the first Chipmunks movie, and there wasn't much of a merchandising presence when the film became a shocking hit. By the time the second film rolled around, there were plush toys, fruit snacks, kid's meal promotions... same deal with Madagascar. We're on the third movie now, and we have Kraft Mac and Cheese shaped like them. That's a market presence.
So when Muppets 2 comes out, we'll see all the merchandise we wanted to see for the first one.