Just the fact that the fireflies were on their last gasp and working with limited resources and literally one specimen for a cure. The chances are ridiculously low, but so are the odds of a fungus based apocalypse and a father who lost her daughter ending up escorting the only cure for all of humanity who was also the same age range. So are the odds that there was only one doctor alive in the entire world who could potentially figure it out. I'm sure a lot have written about it in reviews and such and AI is gonna pick up on all that.
The thing is, it's all make believe, so we should either accept ALL of the premise and engage with the moral dilemma or reject all of it. "Well, they wouldn't have found a cure anyway" is a cop out answer that tries to avoid the trolley problem altogether in defense of Joel's actions. It acquits him with no trial, which is the easy route for people who don't want to actually contend with the core of TLOU and just want to be a fan of Joel cuz he is the best daddy in the whole wide world (which he is, lol). And that's fine, everyone's free to do whatever, but it's silly when they pretend to have done any objective analysis of the subject.