But does Avatar really have the staying power required to dedicate a whole land to it? I find this highly doubtful.
This is the thing that I am struggling to understand, and I share your doubt.
WWOHP probably will have staying power because the books are likely to continue to be must reads for pre teens and teens for years to come. This will mean that the movies will also have staying power. Much more staying power than anything Universal has ever had before, and more comparable to typical Disney movies that are based on classic tales.
Avatar was a good film imo and visually stunning - consider though that part of this was because it really introduced 3d movies to the American public. This certainly contributed to it's huge success at the time, and probably will be a reason why the next films will not be nearly as succesful because the novelty has worn off. This is probably similar to the first movie shown in color (Wizzard of Oz '39) - I'm sure that was a huge deal at the time it came out, but can you imagine a theme park land based on that being popular today?
Because of this, I agree with Mesaboy that this seems like a possible big miscalculation by Disney.