Granted. The Toy story trilogy is a good one (though I'm still wondering what happened to Barbie at the end of it. I mean the day care seems like it will still be pretty controlled but by whom?). And there are many other good trilogies, but from my experience many sequels haven't really lived up and have possibly messed up a lot of their predecessors, seriously look at what Dreamworks did to Shrek? Sure the movies were fine when watching them but it just went weird. Then there is the Saw movies which just got really weird.

Though you are right. I shouldn't jump to conclusions, I just really never saw what they could do for more in a sequel (kinda hoping Ralph doesn't get a love interest or something.)