This I agree with. Even if it worked perfectly this is a global outbreak and distributing a cure worldwide when there are not even working planes anymore is next to impossible.Every person in right mind can see there is no fucking way in apocalypse you can create vaccine, do tested trials and then distribute to all people to “save humanity”. This is big fucking task even fir full functioning government, let alone small group of rebels.
That being said I do not believe the fireflies are evil for hoping against all hope that something could save the world. Neither them nor Joel are evil and I think many people have an issue with a story that asks more questions than it answers. Even if the cure only worked in North America it would make life a whole lot better for many people and they could get on the radio and share with the rest of the world how to do it themselves. It would be a start that could blossom.
On the other hand no parent in their right mind would let their child be used as a guinea pig for what is essentially a crap shoot.
Also in the real world people do brain surgery all the time without harming the patient so why didn't they at least try to do the same with Ellie and just extract some of the fungus without taking out her brain?