The only way it would make sense to build now is if construction prices were low enough that the cost would pay for itself in fuel savings in the coming years.
I have no idea how much it would cost to extend the monorail, everything from 1 million to 300 million per mile has been thrown out on these boards, but it would seem now would be a good time to get a contract locked in place because the contractors are probably looking for work with the economy turning south. With municipalities cutting public works projects and homebuilding grinding to a halt it seems like a buyers market.
If the current low construction were factored with a sharp increase in oil prices and taxes an expansion might make sense.
But Disney would have to be seriously convinced their bus fleet expenditures would be skyrocketing soon to make such an outlay, and unless they have a crystal ball or insider knowledge of OPEC I doubt they would make this move.