Another series most people would probably prefer to forget is -
The first film had a good script and therefore a good reason for existing. It was a touching story and I loved it when it first came out, and was hopeful for the future of Blue Sky Studios, a studio I had previously not been aware of. They did a really good job on the first film, with great characters and a really funny primitive squirrel thing... but then they made a sequel. And the sequel has no good reason to exist other than it is a safe bet that it would bring in money.
And then they made more, and they weren't even consistent with the stories they were writing. The original showed early mammals in the pleistocene era interacting with early man. The second film had the end of the Ice Age, the third film brought the return of dinosaurs, the fourth (sigh) caused the break-up of Pangaea. Again, I'm all for suspension of disbelief in order to tell a good story, and animated films with all sorts of logical inconsistencies are nothing to get worked up over, but those sequels existed only because they would make money - being so inconsistent with their stories showed how little they cared about what they were making.
I was hopeful when they announced Rio that they would finally move away from the now ruined and sequel-ridden Ice Age series, and it was an alright film, nothing special, which was a tad disappointing - but hey, they were doing something different. And yet, what was their next film? Rio 2, another film - and another sequel - that had no reason to exist but to make money, because sequels are cheap and easy. Sigh... I understand industry pressure to make money, but I really wish studios would put more care and more imagination into what they're making, and not be afraid of breaking the mould.