Frontierland has expansion pads available that don't require losing the River.
Disney already has too bad a habit of closing foundational attractions in favor of rides that start and end in a short burst. I'm not sure why anyone would assume the River's "ridiculous amount of space" would be put to better use by any new construction than what we have now.
Wasn't it last rumored that they had considered building a Rapids Ride like Kali in that area? Would you really want to give up the quiet beauty of the River, Tom Sawyer Island, and the traveling Liberty Belle Riverboat for a 5 minute-long Kali River Rapids 2.0 that uses all that space?