The first Harry potter book came out in 1998. It's safe to assume the age of the book's target audience was ~10 years old. In 2018, when Avatar will be opening, that same target audience will be in their early 30's - raising a new generation of Potterheads.
Disney always thrived on the nostalgia factor of the older generations bringing the newer generation to see and share the magic they grew up with. Not only is Disney going to lose in the "cool new attractions" department, they are also eventually going to suffer in the nostalgia one as well.