Here's an interesting take by UCF Hospitality professor Bill Zanetti....
It’s hard to see from the art, but there are new ponds, streams, and waterfalls that are being created, so it’s not a total loss of water. There will actually be more kinetic energy with this than there currently is in the area. But as for why…
Remember that the Rivers of America at WDW are connected by a lock to the Seven Seas Lagoon. Major challenges exist to this river system and drainage in the area that need to be accounted for. The foundation of the river isn’t in great shape, nor is it filtered water. The nature of this river surrounding the island system also prevents construction on the islands. To retain any of this river as is would be impossible, as the connection would need to remain near Thunder Mountain to the lock.
One of the key drivers of this re-do was requested by the park management itself… to eliminate dead ends. That can’t be accomplished while retaining the navigable river system. So you have an aging river basin in dire need of resurfacing, logistical nightmares to improve the islands, and dead end pathways on two sides. Plus access must be created to get to the other side of the river systems for any expansion.
I can tell you that countless rides were taken on the riverboat by key people involved in this decision and it was not taken lightly. They know they have to blow everyone out of the water with what gets built on the site. (Pun intended) It still makes sense.
Cars being the IP was selected for WDI by others. Cars makes ridiculous financial and demographic sense. It sells merchandise like no other franchise, it will eventually allow for the removal of the Tomorrowland Speedway on the other side of the park to free up more valuable real estate in the park, it appeals to the Floridian demographic, and it appeals to families (not single millennials, if they were going for that demographic they would have themed the entire area to A Goofy Movie). Cars also is a friendly story that will provide the friendly alternative to the scary Villains land beyond.
How to fit Cars into the region is actually very real, as JL did concepts for the original films going through places like Yellowstone, but were never realized (though the adjacent franchise, Planes Fire & Rescue does take place in a Yellowstone-like national park). There is a lot more to this concept and I think you’ll see some more Cars related IP coming around the bend that will even cement how it makes logical sense being there. This concept is adjacent to National Parks (not inside one) and it is very pretty. I do have doubts about the reliability of it being outdoors in Florida, but that’s besides the point here.
The aesthetics of the land are based on the Grizzly Peak Recreation Area in DCA. (Piston Peak instead of Grizzly Peak) Obviously there are some aesthetic differences and it’s a Cars ride instead of a rapids ride, but that’s a product of the MK having Tiana’s so close and the IP requirement set above.
In the end, there was no way to save the existing river system. The new lands have major elevation changes that can’t be accomplished without altering the river, and the reality is that they have an obligation to push for more capacity in the park, even if it means at the cost of something so big. It will be tough to see for the next 5-10 years, but I’ve been assured it will all be worth it, and the new water features will be much cleaner and feel a lot more fun.
I hope that explains a lot of the decision making that went into this. Happy to answer a few other questions if you still have them (on twitter).