After the shitshow that is COVID (and still is), I think the writers had is spot with the story line. Even if the Fireflies were to successfully create a vaccine, propagating it around the world and ridding it of disease and the human factionism is going to be a tall order. There will be non-believers of the vaccine (won't get vaccinated), do anything to prevent it from being distributed, or just believe that it is NOT going to save humanity. Whether it be for political, ideological, personal, or religious reasons rolling out a cure is nigh on impossible.
The whole game, I felt tense when it was meeting or fighting other people (Winter chapters) but NEVER the zombiies. Crazy Bill was right about who to fear.
Ellie's death even if a vaccine was to be created would be all for naught because humanity cannot save it from itself. If I knew I had the antibodies, I wouldn't sacrifice myself for humanity... its a lost cause.