I don't think anyone is doubting that Avatar 2 and 3 will make lots and lots of loot. The question really is, is this a property that has legs? Is this a property that will draw crowds for years to come?
Im sure when Avatar land opens it will be huge, but what happens in 5,10, or 15 years? Unlike Potter, the franchise is not as well proven, and does not (currently) have the same kind of fan base. So yes. it IS a big risk to do a "whole land" based on this movie series that isn't even complete.
i will give you a perfect example of why this could be bad...how many rides have Universal replaced because they no longer hold current generation's interest? Most movie/TV properties are NOT timeless. Ghostbusters, Back to the Future, Hanna Barbara, Jimmy Neutron etc have all been replaced by more modern popular entertainment properties.
The Matrix comparisons are sound, if they announced a Matrix land what would your over all feeling be?
Pirates is not a great comparison as the ride existed and was already a classic before the movies. Same goes for Star Wars.
Doesn't the great movie ride even seem dated?