They do have a plan in place. You can read about it here:
https://en.wikipedia.org/wiki/Walt_Disney_World_Monorail_System#Emergency_evacuation
The reason no one was evacuated was because it wasn't an emergency. Removing people from a beam roughly 40 feet in the air is a last resort. Lots can go wrong when you slap a ladder up that high and ask all manner of physically fit (and unfit) people to climb down. The safest option in the event of a non-life threatening stoppage is to keep people in the monorail until the problem can be resolved or the monorail towed to a station. But to claim they don't have a "plan", or that it is preferable to climb down an open ladder 4 stories, or to hike a few miles along the exposed beam 40 feet in the air to the next station, as opposed to waiting in the cars for assistance stretches belief.
The Mark VI cars have ventilation slots that run a significant length of the car. While they will get hot if you are stuck inside them, it's not like being in a car with sealed windows on a hot day. It's not going to be comfortable, but it's still safer than the alternatives. Especially since to get to the beam you have to go out the roof of the monorail, climb or crawl along to the front or back, and then shimmy down a rope draped off the front or back cab to GET to the beam. You can't exactly go out the doors and step on the beam, it's underneath you!
Honestly people. Unless your monorail car is on fire, you are way safer waiting aboard for a few hours than any other option available.