Maybe, but there will still be some sort of merchandise that will sell regardless of the movie. I think Disney will do fine.The problem isn't if the rides will be successful or not, I'm sure AK will see a big spike in attendance. The problem is a possible loss (possible huge loss) in merchandising revenue. The impact won't be Avatarland, but future development.
If Disney has to rely on mostly park ticket sales because there is no new merchandise, the ROI on Avatarland may have just jumped from 5 years to 10 years.