Whether you like Avatar or not, the question of whether it's a good business decision for Disney boils down to one thing... will it attract more visitors, thus generating more revenue.
This is where "cultural significance" or "cultural phenomenon" status comes into play.
I'll throw out some quotes or even a few single terms and I bet you can tell me where they come from:
May the Force Be With You.
Beam Me Up, Scotty. (I know... he never actually said the phrase quite like that)
I'm the King of the World!
Frankly, my dear... (I know that one is actually misquoted, too)
Follow the Yellow Brick Road
Play it again, Sam (another misquote, I know)
Quidditch
One life ends&another begins.
Hi-Ho, Hi-Ho...
Phone home.
Oh, wait... was there one in there you didn't recognize?
Cultural phenomenons become ingrained in our every day lives and speech. If people are not familiar with Avatar (that's that 3D movie about blue aliens, right?), it's not going to attract as many visitors as something that had a proven track record.
I'm sure there were a few outliers who didn't "get" Universal adding Harry Potter. But the vast majority of people greatly anticipated the addition because it HAD cultural significance. With Avatar, you have a much smaller fanbase and loads of people who don't even have a clue what it is. That's not going to generate the kind of buzz needed to attract millions more visitors a year.
Avatar is not an integral part of modern pop culture. It was a flash in the pan (a big flash to be sure), but making a lot of money does not make it a cultural icon. Maybe the sequels will succeed in cementing Avatars place in the pop culture... but as of now it's a big gamble to assume that millions will flock there to see Avatarland. They could be left with Ishtarland!