What if the baby is a fluke that can't be replicated?

We already heard that babies became rarer and rarer before dwindling to Baby Diego was the last one. It wasn't just something that stopped abruptly one day. Which implies that the odds of a successful pregnancy have by this point dwindled to one in ten trillion or whatever, and Kee just happened to hit that lottery.

There's a strong sense throughout the film that if the baby can survive the horrible ordeal the Fishes have put them through, she will be the key to allowing scientists to solve the problem of infertility. Or maybe her birth is a sign that the problem is starting to recover on its own, repeating the earlier process in reverse where it will be gradually more and more common for women to get pregnant.

But there's no explicit reason to be guaranteed this based on what we see. What if the process can't be repeated? We already had the implicit understanding that the youngest characters we meet, who are at this point in their early twenties, will a few decades later be the last remnants of humanity and have to go into old age without anyone younger around to help take care of them.* But what if there were one single woman 20 years younger than all of them? She would increasingly be called upon to shoulder a great burden of caring for them, although I suppose it would be compensated for by being doted on like Baby Diego in her early years.

Or what if she and/or her mother are the only humans capable of reproducing? Would she be forced or at least pressured into being a broodmare? What if Kee is the only one who can get pregnant, and scientists can't figure out how to replicate the process? If all of her children are fertile, there would obviously be a huge desire for her to act as a sort of Eve, even if that means incest among her offspring. But what if she can have kids but then they end up being infertile? Is it even ethical to have children who are going to end up being a group of siblings that are 20 to 30 years younger than the rest of humanity? That seems pretty screwed up, but I can also understand the instinct to try to keep running out the string as long as possible.

*Which would be an interesting scenario to see play out in its own right — I feel like I've seen some mention that there is a book or movie or graphic novel or something along these lines, which I would be interested for sure in checking out if anyone knows what I'm referring to.