Pop was not built post 9/11, it was in the process of being built when the 9/11 tragedy occurred. Tourism came to an almost grinding halt. There was a huge decrease in attendance to WDW. Therefore, continuing to finish all of the buildings wasn't feasible.
The Treehouse Villas were run-down and unusable. For a while they were used for interns. Yes, they are on wetlands and there are strict federal regulations regarding building on wetlands. They are now being rebuilt in pre-fabricated sections. It is a really cool process the way they are bringing in the pieces of the buildings and placing the together with cranes. They will be part of SSR.
Discovery Island was once available for tours. However, it is now a bird sanctuary, which it kind of was when open....Birds are very dirty and carry many contagious diseases. The island is not suitable for humans anymore.
River Country was the best. We went there as kids. It was the first Disney water park. Wet 'n Wild was also around, but not owned or operated by Disney. River County too was closed. Rumors are that the water was no longer deemed safe for humans to be in. Typhoon Lagoon was already open when RC closed, so I don't know how the attendance figures were at RC or if that had anything to do with it closing.
Regarding not building Pop and continuing to build DVC resorts. Do you have any idea how much money people are paying to join DVC? They are essentially paying for the building of those resorts. Renting rooms at a value resort will take much, much longer to pay for the construction than the sale of ownership in the DVC resorts. Minimum investment for a new DVC contract purchased through Disney is over $16,000. Add to that annual maintenance fees, and these resorts are much more feasible for Disney to build than value resorts.