Avatar is one of the biggest outliers of this ever. It's why so many people thought an Avatar land was a bad idea. It left almost zero pop culture footprint. A lot of people laugh at the idea of stuff like Halloween costumes. But it is a decent gauge of how something is trending. Like you said, it's not the end all be all, but it does give some good insight in my opinion.
And even if you don't look for Halloween costumes.
Toys. Kids toys for this did not sell the way Marvel, Star Wars or even Ghostbusters did.
Even at the park food and merch is kind of there, but not a must have or kids begging for items.
It rarely even feels like THE REASON people go to Animal Kingdom anymore. As happy as they are to ride Flight of Passage, its already lost the anchor feeling that Star Wars and Potter have.
The budget is going to go up, and the diminished returns will continue until it is just another tent pole, not the magnificent that they want. I think the stench of 2 honestly lingers with a lot of people.