Pretty sure the Fireflies were nearly wiped out anyway by the time you get to the end of the game, as well as more or less just being a bunch of terrorists in the first place. No way they'd ever be able to scrounge up proper resources to both study as well as mass produce any kind of cure. Or even have the clout necessary to have people accept it.

Really, the major flaw of the first game is that they do a shit job at not making Joel 100% in the right. On surface level, sure, it looks like he selfishly fucks up, but there are plenty of things going against it. All in all, the real meaning of the game is/should be about saving the world through human connections, not some magic cure-all. Shown through Joel and Ellie developing a strong bond, despite Joel's past making it difficult.

Plus, realistically speaking, the mushroom zombie stuff would've killed itself off eventually. (It just doesn't because of more bad writing.)

Mushroom zombie should have killed itself off incredibly soon. Like, mushrooms arent exactly energy power houses, 15 years after the first outbreak every single zombie should have been dead for a long while.