Suppose Disney had instead announced plans to build a new land at AK based on fantastical creatures from a fictional foreign world. It would be a planet invented by Disney, with creatures nobody had ever heard of, born from the creative minds of Disney Imagineers.
Would Disney fans get behind the idea? I think they would, especially if Disney pledged to invest heavily in the new area. While some would no doubt still long for the unicorns and dragons of Beastly Kingdomme, I think most would support the idea of a portion of the AK park dedicated to fictional, otherworldy creatures.
Enter Pandora, the fictional world created for the "Avatar" movie(s). It's basically the same thing described above, with the exception that Pandora was conceived by James Cameron, whereas the above idea would be solely from Disney. Just because the original idea came from outside Disney, does not make it a bad idea.
And just because the movie was mediocre, does not make it a bad idea. Splash Mountain is a fantastic ride, but it is based on a racist movie nobody has ever seen. The Jungle Cruise is a Disney Classic, but few even realize the ride was inspired by "The African Queen." Other well-loved attractions, such as Space Mountain and Pirates of the Caribbean were not tied to movies at all.
So my point is that the quality of the movie has very little to do with the quality of a theme park attraction. An interesting idea, with the right creativity, along with the right financial commitment, can make for a great attraction. The Disney parks have demonstrated this time and time again.
A Pandora-based land might not have been my first choice either. But I'm willing to give it a chance.