I think it's highly possible they had some sort of contract for the Avatar IP that made it difficult to back out. In fact I've always figured James Cameron's motivation in all this is to build the perception of Avatar as this Harry Potter or Star Wars type franchise. I think this was heavily used as bargaining to get the production of the other movies moving along (which even still seem to be suffering from setbacks), this enabled James Cameron to reference the fact that it's going to be a theme park land to give credibility to the idea of Avatar as a franchise not just a one time hit. I think this is the reason Avatar land was announced so early, I suspect James Cameron pushed for the announcement. In short I think Cameron was/is playing the studio and Disney against each other as the reason Avatar will be so great. They also would have worried about how the cancellation of Avatar would have looked. How would stockholders have reacted to the cancellation of The highly hyped Avatar, especially if that happened before the announcement of Star Wars land.