It basically comes down to whether we want to rigidly adhere to some 75-year old concept of what is "appropriate" for each park vs allowing a little flexibility in the interest of expansion. I don't see Indiana Jones at DAK as any worse of a fit than Kali River Rapids or Expedition Everest. The landscaping and architecture certainly fit. There's a similar element of adventure and exploration. IJ will certainly have some animal presence.
Is GOTG a perfect fit for Epcot? No. But they massaged the storyline and presentation to help. Specific to this one, Disney basically had two other choices: use the same ride and make it some generic scientific theme which was a "better fit" or put the ride in a different park. To the latter point, we can't keep shoving all the "good stuff" into Hollywood Studios. As for the former, there's really no doubt that the GOTG overlay generated much more excitement and interest vs some generic big bang theme.
It's tough to wrap my head around the idea of Rivers of America becoming a fully developed plot of land. That said, all of these things are also true:
- Functionally, Tom Sawyer Island and the riverboat hold almost zero value to the masses. The hourly capacity of two Cars attractions will certainly exceed the number of guests riding the boat or walking around the island in same amount of time.
- Having a shorter route from Fantasyland and Liberty Square to BTRM / Tiana / railroad will be a welcome addition for everyone.
- If I want that island / riverboat experience, it still exists in
Disneyland. And that's the one that Walt actually created.
- When I'm standing in the middle of this new land, seeing BTMR in one direction, Frontierland in another and Liberty Square in another, the theming will probably be a decent fit. This isn't supposed to be Lightning McQueen and Radiator Springs, it's a different time and place in the Cars universe
I've been a fan of the Disney Parks for many years and enjoy things like Haunted Mansion and Everest which don't have specific IP attached. But overwhelmingly, I suspect the general public wants to see their favorite characters come to life. When you're spending $250-500M on a new attraction, it's a lot easier to market when you've got that built-in name recognition. (Not to mention the merch sales.) Disney's competitors aren't building non-IP attractions for the same reason..unless they're some sort of generic steel coaster where exactly zero dollars are being spent on theming.