Back when The Walking Dead was picking up steam on AMC, it was largely due to the popularity of the Zombies craze it jumpstarted. Serialized horror wasn't a mainstream genre for television shows back in 2010, despite The Walking Dead largely focusing on elements of drama. The zombie apocalypse fad that The Walking Dead started was tangentially responsible for assisting in The Last of Us' major success as well, despite being wholly different in comparison.
However, a potential The Last of Us 3 would face a similar problem that The Walking Dead has already seen. Assuming there is some kind of sequel or separate story in the same universe, The Last of Us Part 3 will almost certainly reach a time where the infected have been around for almost half a century. Which begs the question of what a The Last of Us world looks like 50+ years after Outbreak Day? There could be a world where Ellie's immunity doesn't matter, or matters more than ever, if the infected eventually decay and die after all this time.
The Decaying of Infected in The Last of Us
For those who haven't watched The Walking Dead, but are fans of The Last of Us, there's an interesting phenomenon with the "walkers" in The Walking Dead that The Last of Us has yet to encounter. As the show ends in its impressive 11-season run on AMC, over time viewers begin to notice that a lot of the undead begin to decay and rot. While most undead in The Last of Us are either running at full speed or mutated into something even more gargantuan and dangerous, there's not many instances where Clickers or other Infected have decayed to the point of inability.
Most of the time, that kind of undead entity in The Last of Us has been depicted as overgrown Cordyceps ingrained into the wall. Occasionally those infected turn into jump scares in some of the cramped corners of The Last of Us' world, but they've never been like The Walking Dead's walkers. The infected are always a threat in The Last of Us, no matter how young or old they are, whereas decayed/rotting walkers in The Walking Dead are comparatively easier to dispatch. A similar amount of time has passed in both universes up to their latest points, but it does lead to a larger discussion of what The Last of Us would be like if the infection was no longer as prevalent and dangerous.
A Post-Outbreak Society in The Last of Us 3
Granted, the first entry in The Last of Us series was largely about the threat of the infected, which makes sense as the zombie apocalypse game's first outing. Joel and Ellie need to take a cross country trip and avoid plenty of infected along the way, even though there are several human threats along the way. Though now, in the wake of The Last of Us Part 2's events taking place around 25 years after Outbreak Day, there hasn't been any physical sign of the infected becoming weaker or more frail as a result of decay/rot. The infected have always been a pervasive threat in The Last of Us, but is there a world where eventually the infected are no longer as strong?
In terms of a potential The Last of Us 3, showing off more infected starting to die off due to rot or decay could be an interesting internal conflict for Ellie if her story continues. All throughout her life, the Infected have posed a very visceral physical threat to her, but she's never worried about the airborne Cordyceps infection. Is there a possible world where her immunity to the Cordyceps infection even matters if the infected do eventually rot and die on their own? Infection sites could be demolished or destroyed, and theoretically problem solved. Now imagine how Ellie would feel after knowing Joel lied to her, and then eventually discovering her immunity may not actually matter after all.
An Alexandria-Like Society in The Last of Us
There's no sign of that happening at the moment, but in a sequel like The Last of Us 3, the subject would likely need to incorporate the rebuilding of society in some way. There are several sub-narratives in The Walking Dead all about attempting to rebuild modern society, like Alexandria. There was some hinting of this in The Last of Us 2 with the Jackson settlement, but since the narrative focus was largely on Joel, Ellie, and Dina, players didn't ever learn a whole lot about how Jackson survives day-to-day when the Infected arrives.
The Last of Us, in both games, has already tackled with focusing the narrative away from the infected, but what does a future game in the franchise look like when the infected becomes less of a pervasive threat? It would be interesting to get some more insight on the survivors, what they're doing to expand, survive, and thrive in a world where there's less Cordyceps abominations attacking the living constantly. How would that future affect Ellie, who's spent most of her life fighting and escaping the infected despite being immune to Cordyceps itself? Perhaps The Last of Us 3 could shed light on that kind of story, even though a theoretical sequel would likely be years away.