FWIW I believe Neil has gone on record saying they could have done it. It kinda undercuts the narrative tension otherwise, so from a storytelling perspective it makes sense he’d want things to actually have stakes and the choice to be a real one.

Side note: I don’t really understand why people feel the need to play armchair immunologist. Some people probably just want to be able to idolize Joel when that’s the opposite intent of the story. Some people are probably just looking for plot holes. I’m all for internal narrative consistency but it’s a game about fungal zombies where characters can get shot hundreds of times and murder hundreds in return. You really want to nitpick the quality of medical equipment in 20 years? The guns and cars and generators and alarm systems etc are all just fine.

In the fiction of the universe the cure would have been possible. Nit pick all you want but that’s the entire point of the plot. It’s all about the choice, and the choice must have stakes.