No, that's not how heat exhaustion works. Plus, those outside in the sun without cover are constantly being fed in new energy. That's why the idea of being in shade/cover is so important.. and why the gondola's design is so important. It's not a simple glass box.
Leaving the queue is not easy to do in a long extended queue.. and even those that do it is a very small percentage.
The point is literally THOUSANDS of people would go through this trial and tribulations without incident every day. Maybe, just maybe, it's not as extreme as you paint it to be.
Yet, you still see thousands of those high risk people waiting in the sun for parades, etc
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Yes, some people are more susceptible to heat issues... but those same people are going to be at risk no matter what form of vehicle they are in if they do not have AC. So there is risk of this everywhere. You have to manage it and you can't account for every possible extreme. For those extreme cases, or someone in distress, Disney can evac those people first.
The problem is people are exaggerating the situation people will be in.