OO and the planet in general seems unfit for any resurgance of a global society. In between monsters, magic, and celestial beings roaming around to cause havoc there are just so many different species with their own kingdoms and realms that it's unlikely anything will reunite the world. Much less humanity.
PB is perhaps the most imperially minded character of them all but after the Gum War and Marceleine probably ended up just settling for control of her own city rather than expanding outwardly. >>102758630
It is a bit sad that he was the last true human adventurer. But then, that's a problem for us and not for him. He lived out his life to the fullest, so I guess, for the last great human hero that's the best we could ask for. And he did revive humanity for a time.
Perhaps he even did decide to pass on his genes to some hybrid child and they live on somewhere.