I don’t think the point of this was to guarantee the champions’ safety. While the organisers had certainly made an effort to have this tournament be safer than those in the past, guaranteeing complete safety here is simply impossible. Let us look closely at what was actually said:
“We are going to be patrolling the outside of the maze,” said Professor McGonagall to the champions. “If you get into difficulty, and wish to be rescued, send red sparks into the air, and one of us will come and get you, do you understand?”
Note that she doesn’t say that if you’re dying they’ll save you. She says that if you get into difficulty you can choose to be rescued rather than forge ahead into the difficulty and put yourself at risk of dying. This is indeed what we see when Harry considers this option:
But not one of the spells he had practiced had been designed to combat a sudden reversal of ground and sky. Did he dare move his foot? He could hear the blood pounding in his ears. He had two choices — try and move, or send up red sparks, and get rescued and disqualified from the task.
He wasn’t in danger of imminently dying, but he was in difficulty. His choice was to give up in the face of the difficulty or continue and take the chance that he would put himself in danger.
Similarly, when Harry hears Fleur scream he acknowledges the possibility that she could have been in too much danger to be able to send sparks:
There was no sign of red sparks — did that mean she had got herself out of trouble, or was she in such trouble that she couldn’t reach her wand?
There is no pretense here that anyone could be rescued from any situation. It is apparently understood that sparks would only help you if utilized before the danger really starts.
