As you mentioned, the real question is which park gets cannibalized. When Hogsmeade opened, IoA attendance grew YoY by 28.5%. For the rest of Orlando, MK grew by 1%, Epcot stayed flat, DHS grew at less than 1%, DAK grew by 1%, USF grew by 7%, and SeaWorld shrunk by 12%. When Pandora opened, DAK attendance grew 12%. Elsewhere, MK grew by less than 0.5%, Epcot grew by 4%, DHS shrunk by 0.5%, USF grew by 2% (with the addition of Jimmy Fallon), IoA grew by 2%, and SeaWorld shrunk by 10%. USF & IoA also had a semi-third park open with Volcano Bay. I could continue the comparisons all day, but the data is similar for other time periods so I will make some generalizations. Universal tends to not cannibalize themself and actually lift sister parks, Disney tends to not cannibalize themself but does not lift sister parks, and SeaWorld is where the cannibalization is occurring. How much farther SeaWorld can be cannibalized is unknown, but Universal tends to be in the best shape for these battles.