Again, "fail" is never in the conversation. It's a new attraction in the theme park capital of the world. It'll do fine.
The question becomes can the land be popular enough to justify whatever ransom Disney paid Cameron and Fox for the rights to Avatar? If the land ends up giving the park a similar bump to what we saw with Everest (a million or so), the money shelled out to get the property looks like it's wasted as they probably could have gotten that bump from building an "Up" land, let's say. If it ends up boosting DAK by Potter-like numbers, it's obviously worth it.
I don't think the land will "fail" by any stretch of the imagination. The question becomes whether the expense for the property is justified.
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Eh, DAK's attendance is only 10 million -ish if you believe the TEA numbers, which is an incredibly small fraction of how many people they want to see that movie.