In the Fallout series, most notably Fallout 3, Fallout 4 and Fallout 76, East Coast Super Mutants continue to grow as they age. The FEV mutagen that turned them into Super Mutants continues to change their physiology and while they’re already stupid brutes, eventually it turns them into lumbering Behemoths that are of gargantuan proportions. This worked in Fallout 3 and Fallout 4, because they’re set over two centuries after the world ended and Super Mutants were being created for roughly that length of time - two centuries to turn into a Behemoth isn’t that big of a deal. Fallout 76 changed this, however, because it’s set only 25 years after the nuclear apocalypse and there are Behemoths running around. This latest addition of the Fallout series puts the issue into roughly the same time frame as The Last of Us.
Whether you’re a survivor in The Last of Us or Fallout, you’re logically screwed just by the way in which the creators have crafted the world. Ten to twenty years is all it takes for base enemies to turn into the strongest enemies that’re available, and they don’t even get a say in the matter because it’s a natural process. Every human who is infected in The Last of Us will turn into a Bloater if they survive long enough. In Fallout, every human that’s dipped into FEV will turn into a Behemoth if they survive for long enough. It’s literally only a matter of time in both cases. Since the narratives need these creatures to be the biggest threats in their respective stories, the logical progression of events is that humanities chances of survival continue to plummet the longer the timeline continues. The Last of Us is set 20 years after the world ended, while most of the Fallout series is set centuries after the fact, which is plenty of time in both instances for the vast majority of these creatures to reach their final form.
The thing is, this could actually make your world that much more interesting. Instead of ignoring the rules you write into your own world, you can run with them and create a truly deadly setting that is a serious threat to your characters. In both The Last of Us and Fallout, if things had progressed as they should have, with humanity hiding from their growing problems instead of dealing with them… well, now they’ve got a problem that’s a million times worse. If time is the deciding factor in your enemies strength, then time is the enemy. If simply not dying is all it takes for your enemies to reach ultimate monstrous god mode, something even your protagonist and background characters have managed, then use that to your advantage.
If your zombies or mutants evolve over time, use that to create a story with unique zombies or mutants. If you don’t want your world overrun with final form enemies, then don’t set the story in a time period where that’s the logical conclusion. You’ve still got all the years before that to play with, so go nuts in the safer years. If you want an enemy force that is linked (wherein each creature evolves through each of the different forms) but you don’t want a world that’s inevitably overrun by final forms, then simply don’t make time the deciding factor. Have them get injected with a serum, or have it as some innate biological process that’s triggered when the need arises. However you do it, the golden rule is to keep your world consistent. The less your readers/players/watchers have to question how things work, the deeper they can invest in your world without making excuses for it.