It always depends on how strong the mistake was
Where do you know what is
strong and what not? Sorry to say, but what you said is a tautology. Your input serves as a measure in order to prove the input... that doesn't work out. I believe I understand your feelings, but your conclusion is not entirely derived by logic, rather feelings. As I understand, PBL is looking for the pure logical approach to distinguish between minor and major factors - and the TL in CLB cannot be sorted out to be "primary" by applying statistics or mathematical formulae.
BTW, it is not correct, that TAM 3054 was the only airplane that had problems with the length of the runway. There
were problems, e. g. just the day before. Had the runway been longer, say 2400 meters (see my calculation some posts above), they would live today. To put it in your words: "Because this predominant cause was so devastating that it broke their neck! That's why!"