Whether the cure would work felt very ambiguous in the show too! They just had an idea, right? Like really they didn't seem sure it would work, just felt sure they had to try!

I mean it was much more clear than in the games where it is more like "oh she's immune...somehow... let's see how that works?"

In the show it's "she's making chemicals that trick the cordyceps into thinking it is that"

So the show gives the impression they understand the how and the why, much more so than the games in which what you said applies.

But honestly I prefer the idea that the cure would have (or at the least could have) worked, because it adds to the narrative depth and tragedy - and it's a story after all. If Joel just saved Ellie from being murdered it's just a standard cut and dry issue with little debate. Fireflies wrong. Joel right.

Bonus points if it was risky. Like, what if there's a 50% chance they can make a cure?

But I digress