A centralized HUb and light rail would solve their problems...but they don't "do" projects that accomplish great things any more...
AMEN
A light flexible rail is not that difficult.
Flexibility
The number of cars on a train can be adjusted upon demand. Not so on a monorail. You have 8 cars you have 8 cars.
A unit gets stuck it would be easier and faster to offload the people as it would be ground level.
You can have several main lines with switching to different destinations. For example you can have a main line running from MK to Disney Springs with switch offs (spurs) to EPCOT, OKW, Saratoga Springs. Same with a main line to Animal Kingdom Railroads control switching remotely all the time from a central location, so can Disney.
Cost
Buses are costly to fuel and maintain. Also one person per unit light rail vs one person per bus.
The big cost in a light rail is building the right of way. Part of the cost would be purchasing the right of way, however Disney already owns it. After that it is much cheaper to operate.
Monorail is much more expensive than light rail to build.
Electric motors are much simpler than diesel engines. The number of things that can go wrong is much smaller. Generally they are more reliable
Equipment replacement
With the monorail you have to find somewhere to replace the units thus replacement cost would be very high. With light rail there are many more sources. Replacement cost would be far less (the old supply and demand.) There are many light rail systems out there. The cars are already being built. You would be buying from inventory.
It takes is a little magic and and a lot of vision. If I thought it would help I would send the definition of vision to corporate.