Were God to show grace to all of Adam's descendants, men would at once conclude that He was righteously compelled to take them to heaven as meet compensation for allowing the human race to fall into sin. But the great God in under no obligation to any of his creatures, least of all to those who are rebels against him.
We need to shed our unearthly and nonsocial and idealistic and romantic and uber-spiritual visions of kingdom and get back to what Jesus meant. By kingdom, Jesus means: God's Dream Society on earth, spreading out from the land of Israel to encompass the whole world.
Humanity is not without answers or solutions regarding how to liberate itself from scenarios that invariably end with mass exterminations. Tools such as compassion, trust, empathy, love, and ethical discernment are already in our possession. The next sensible step would be to use them.