The buses are a problem because Disney doesn't have enough running. As for lowering the costs, that's a guess at best. How many cast members will it take to man each entry and exit point, how many specialized mechanics will it take to keep the system running.... I have no clue but I suspect there will be a lot of people working behind the scenes that you or I could imagine. While in theory a continuous people mover may seem better the fact is we don't know nor does Disney know the answer to that until the system gets put in operation. If buses have become too unreliable the it really speaks to the maintenance program Disney is using. I've seen school that operate huge buses systems on a daily basis in a very reliable manner, but then those school transportation systems make reliability a priority, I don't think Disney has made reliability as much of a priority as they have keeping costs down when it comes to transportation but that doesn't mean their bus system could be reliable it just means that Disney doesn't care enough about the system to properly fund it.
I don't know how many CM's that Disney is planning on using for the Gondola's, But, they have buses running to the resorts about every 20 minutes, very seldom is it the same bus or driver. There are at least 1000 drivers on the payroll. (That is a the number quoted to me when I was looking into applying for a driver position over 10 years ago.) Factor in the number of maintenance people required to keep that fleet running, the cost of fuel for 20 ton vehicles, the cost of changing the oil (about 25 gals. per bus), Tires? At least 6 per vehicle at $300. apiece. Constant daily requirements, touch up, probably daily washing. Cleaning crews to keep the interiors clean, brake replacement, the cost of parts replacement, the size of the garage/maintenance facility itself. And you can put a whole lot of CM's at those five stations on the Gondola line. A bus comes with a whole lot of expense that none of us even think about, but, trust me... it's there and it is enormous.
Where did you get the idea that the buses are unreliable, they are fine, but, they are like that trophy wife, they come with a high cost. They are also smelly and messy and are a prisoner to traffic. I don't want to see any bus drivers laid off, that is the last thing and the most important negative to the change over to Gondola's. However, life and times change. We cannot live in the past forever, at some point we must move forward and even if Disney saves a lot of money, they also are, by default helping the environment and thinking outside the box to put in a new and different thing. Something that once was the hallmark of Disney. There is a very big picture that must be looked at and to inject the lack of maintenance scenario is just grasping at straws. The problem, if anything, maintenance related is that it requires a lot more cost then we can even imagine to do well, which it is doing very well.