For me it's simple:
Aviators are invaluable and difficult to train, yet to this day you can read about their deaths in training.
There's no guarantee once in the air and that's precisely why they do aerial exercises, taking that risk prepares them.
In Sousou no Frieren, you must be accompanied by a first-class mage just to travel in the North against demons. Simply put, you must prepare to die.
This exam is a certificate that this mage you hired will survive against all odds and escort your party, not the other way around.
Let's remember that nobody force second-class mages to take the test as well.
I remember people talking about how Frieren's age changed the perception of time in this story. The author is doing the same with the balance of power.
The death of a candidate is no doubt a great loss, but certainly not a waste since that's how you make first-class mages; they are the mark of excellence.