There is simply no impetus for Disney to move away from the bus system--it is extremely efficient and now they are investing in audio for the journey to the parks and even video in the future to make it seem less like a bus ride and more like the Disney experience is 24/7 with no gaps--magic in the parks, magic at the resorts, and magic inbetween--from the moment you arrive at the Orlando International Airport til the moment you return to it, Disney is ever-present.
The only thing I could see that would make Disney move away from busses is if fuel costs went completely through the roof. But at that point I think they'd just buy a fleet of alternate fuel busses.