The choice isn't really "doom humanity" as humanity would eventually evolve some sort of defensive measure against the fungal zombism;
Surely humanity, without a cure, isn't going to survive long enough to evolve.
Uh it is sort of a non choice. Save tons and tons and tons of people while losing one, or save one and lose tons and tons and tons of people. It sucks, but it is also the zombie apocalypse and all that that entails. The answer is obvious, especially when the "one person" is perfectly willing to die for a good cause.
Yeah but that's what I loved about this game. They did such a good job of building the relationship between Joel and Ellie. If I hadn't played the game then the decision would seem simple but after watching the relationship grow I can relate with Joel and understand why he did it, that's pretty cool.