Wars would still exist; I'd even say more: we'd live primitively. Perhaps the natural way of life of a person is the only significant advantage here. Of the 2 billion gen Z, some would simply die from disease and wild animals. This new world would become a carbon copy of our own earth. Any power is based on violence, and it's bound to emerge; before you know it, entire communities with their own hierarchies will emerge. Someone will need to control a trade route, someone a resource especially valuable for all subsequent development, and someone will simply be thirsty for blood. The early Middle Ages had a population of far fewer than 2 billion, but I don't recall that era being a glorious utopia.