The increase in attendance at DAK will almost surely come at the expense of DHS. Epcot will likely be fine because festivals and food and stuff. We'll have to see once they really start pushing Avatar how the response is. It does seem to get covered by mainstream entertainment outlets anytime even the smallest bit of info comes out about it so this land certainly won't open to silence. Will the people be there though is the question.
Also, I don't want to get into it much bc it's OT, but Kong/Potter had much larger gains on overall Resort profits because, there's a lot less at Universal to generate revenue than the entire WDW Resort. What with 6 Parks, 25+ hotels, 100+ restaurants, etc, attractioins/merch don't make as big of a dent in the numbers as they do at Uni. That's not to make an excuse because Kong and especially Potter have done great things for the resort, but it's just fact.