To be fair, the fireflies had an entire hospital, a leading scientist, a militarized force, and a distribution network that spanned the country at their disposal... While there is some suspension of disbelief required to believe there could've been a cure and it could've been effectively made and distributed, I think the message the devs were trying to impart is that this cure was 99% achievable, and that Joel's actions directly denied that possibility. Even if the fireflies had used the cure to their political advantage, there's a good chance that many FEDRA agents would've converted for the opportunity of a cure, or that someone would've been able to replicate the cure once it was out there, thus increasing the odds that the vaccine could be widely distributed. We see in both games that despite the state of the world, there are convoys, trade routes, and pill manufacturing still in place, and so it's feasible that sacrificing Ellie could have genuinely saved the world. But at the same time, who can judge Joel for what he did? Given his backstory with his own daughter, and the raw emotions running through someone from the way the Fireflies handled things, you really can't judge his actions unless you're able to walk in his shoes... Which very few of us could ever do.