Unless Bombardier suddenly went out of business Disney can easily contract with them to provide necessary parts.
Bombardier Transportation is still around. However.....
I doubt the original dies and tools are still around after almost 30 years. Disney might be able to talk Bombardier into building parts, but I don't think it would be easy or cheap.
Correct. Usually what happens in that business is that once the contract is fullfilled, unless additional orders for the same type have come in, the assembly line is shut down for good, never to restart. So if Disney went back to Bombardier and said "build us monorail train sets like you did xx years ago", Bombardier will tell them that they can not build them the same exact monorail train set, but they can build them completely new ones. Of course, since its low volume and very unique (limited to no opportunity for other sales) production, its going to be very expensive.
And, depending on how unique the parts for the monorail system are, it may become more difficult over the years to acquire replacement parts. You can get companies to make new parts, but again, its a one off and very expensive.
Light rail, although expensive to build (the last figure I heard was $1 million per mile, probably would be much more at Disney World since its elevated) would be much easier to acquire train sets, cars and spare parts, since additional train sets can be added to existing contracts. Additionally, light rail trainsets are not a unique system, used cars and engines are available for refurbishment. Not to mention, there are always going to be companies selling light rail train sets, so you can always acquire newer, different models by adding an order to an existing contract. You don't have to buy the exact same thing.
Don't forget that NYC had an elevated rail system in the 19th century, and Chicago still does. The technology and engineering isn't impossible, its just going to cost money.
FWIW, Wikipedia says the monorail system at WDW is the 5th largest in the world, roughly 23 miles of monorail track. Numbers 1 through 4 are in China, Japan and South Korea.