Avatar IP, maybe worth about 1 billion in revenue.
Harry Potter IP, maybe worth about 27 billion.
In terms of sticking power, something that generates 27 times the revenue is going to have a much better staying power in terms branding. Disney typically invests in things that have stickiness in terms of IP. That isn't to say that it couldn't be amazing, or fantastic, just that Avatar doesn't lend itself to the typical IP traits that has created successful brands.
Example. Star Wars is the kind of IP Disney does well with. Marvel is the kind Disney does well with. I don't see the attributes of Avatar and how it fits. Disney can create amazing spectacular areas and themes and rides. But that doesn't mean there aren't better ways to spend $500 million.