It's easy to overlook because Avatar was a horrible movie and the only reason DisneyCo pounced on it was because JK Rowling shut down their plans for haphazardly tossing Harry Potter into the Magic Kingdom.
They needed a quick and popular cash cow to compete with the boy wizard, and James Cameron was like "Hey, I've got this franchise I'm starting! The first movie made TWO BILLION DOLLARS! We can work together!"
And then this happened:
The four upcoming sequels to James Cameron's 'Avatar' have been pushed back once again. If there’s one thing that’s been entirely consistent about the 'Avatar' follow-ups since their initial announcement, it’s that they’re entirely inconsistent in terms of release dates. And, for that matter...
www.hollywoodreporter.com
Whoopsies. Counting chickens before they hatch.