With any expansion of the monorails, more trains would definately be neded. On most days, at some point in the day there are 11 trains out on the beam. When you factor in that there is often one train that is completely out of service for maintenance overhaul, then that is your current fleet. Several days in the past weeks they have been running whats known as 4-4-4, which means they have 4 trains on each beam (resort, express, and Epcot). There are times during the day that they may go down to as little as 3-3-2, but that is usually only in mid-afternoon, so that they can do some of the daily maintenance on the trains that they might not be able to get to due to late park hours.
The other thing that is not mentioned often when talking of Monorail expansion, is what happens when trains break down. When a train breaks down, you lose virtually all capacity on that line for however long it is down. If you were running 3 trains, that is a loss of potentially 900 people about every 20 minutes for what is often an hour or more. On the flip side, when a bus breaks down, it is a potential loss of 70 people, for just as long as it takes to put another bus out there. If guests are stuck on a broken train, then they are stuck on a broken train. If guests are stuck on a broken bus, another bus pulls up next to it, they move to the new bus, and off they go.
It may not be pretty or exciting, but buses make the most sense for the operation Disney has.