There is a damn good reason why there is no fearless rune. Because fear is a natural survival instinct that keeps your ass alive! Fear is what keeps people from doing crazy and reckless things. For the military, law enforcement, and emergency personnel fear is essential because it makes you think twice before you throw yourself impulsively into situations. However, what makes them heroes is that despite their fear they often go into dangerous situations anyway. They simply master their fear, not completely get rid of it. As the great Nelson Mandela once said:
I learned that courage was not the absence of fear, but the triumph over it. The brave man is not he who does not feel afraid, but he who conquers that fear.
And the thing is, no one not even seasoned Shadowhunter Luke brings this point up at all. Without thinking, everybody (including the fucking adults in the room) just jump to experiment with this rune. A rune that if ever used in a battle situation that can get people killed.