Disney Seas was in development for a long time before they entered the construction phase. As I remember, DCA's development happened faster than Disney Seas, and the result (DCA 1.0) wasn't so good, neither was the lackluster Studios in Paris. Also, Disney Seas is not owned by the company . . .
Yes, Tokyo Disneyland opened April 15, 1983, Epcot opened October 1, 1982, but a couple years later the company was almost picked apart by corporate raiders. Epcot lead to hemorrhaging of money, and Disney didn't want to do Tokyo Disneyland, but they got a sweet deal and didn't have to pay a dime. Unlike Tokyo Disneyland, Disney does have to pay for a good chunk of Shanghai Disneyland, and it is very different from Tokyo Disneyland.
Should Disney do Avatarland, Carsland, and Shanghai Disneyland all at the same time, it will be a big deal financially and creatively to pull it all off, IMHO. Because Shanghai is a go, don't see Avatarland and Carsland DHS being built simultaneously at all.