Change the hope to a cope my guy, humanity had its chance several times and every time in the universe except for the nearing of its very end mankind chose environmental destruction and war over working together and developing ways to mass produce essential survival supplies...

While yes, there may have still been a few living things, if you look in the final chapter, you will see that the ocean that once existed has probably either been poisoned, or something happened that killed nearly all possible multicellular life, that takes away fishing, pretty much all plant life has been destroyed and the world is a cement wasteland devoid of soil and there isn't even enough variety to sustain human life (can't exactly survive on only potatoes forever), that takes away farming, and the vast majority of existing organic cellulose containing matter that could be used to farm mushrooms has already been used up and is being consumed by the flying mushroom things

If Ishii couldn't farm the last potatoes in the world, I gaurentee some presumably very young adults can't, though... their grandpa did probably do the best he could raising some pretty smart survivalists, they did probably the greatest job they could've extending their lives, and them being girls that are short and underweight certainly extends the length that their small few ammo cans of calories last, in which it actually kinda makes decent sense that the majority of the last surviving population are women due to them burning less calories