Nothing is 100%, but I seriously doubt Disney would ever get rid of Fort Wilderness. As some people have mentioned, it has a very high occupancy rate, and is one of the original Disney "resorts". Just because it doesn't cost $300 a night doesn't mean that it doesn't make money.
Once again, as other people have mentioned, there is very little cost with Fort Wilderness compared to other resorts. There electric bill is probably what, 20% at most of other resorts, same for water, etc... There is a also a much smaller staff that has to work the campground.
Now, of course, there are other costs that might run higher at other resorts (other resorts probably make a ton more money on food/drinks/etc...). As well, there is a much more elaborate bus system at the Fort than most resorts.
All that being said, I just don't see it going anywhere. If Disney needs to build another resort, they're not going to tear down Fort Wilderness to do it. They have plenty of room elsewhere to build, and it would actually be a very dumb business move to wipe out a high occupancy area that makes them money, simply to put in another money making resort. Why not just have both and get the profits from both resorts?
That, and the fact that Disney has no intention of building any new resorts themselves anytime soon. Their occupancy rate still isn't high enough to warrant it, and it would just be a gamble on their part. And if you think Disney is going to allow another resort that is non-Disney to be within eyeshot of the Lodge/Contemporary/the Lake, I seriously doubt it. They still have TONS of land on the outside edges they can annex for this exact reason.
This new resort isn't going to be right on top of Fort Wilderness. Chances are, you probably will never even notice it. Don't worry, we have many more Hoop Dee Doo's and Campfire Sing-A-Long's to come
