I am arguing on the basis of
classical theism which declares that God knows everything that happens, happened and will happen, but also the “variants” of what “would happen, if…”. You can look it up in the dogmas, if you don’t believe me (
here, under I.27, I.28 and I.29). As such it is perfectly reasonable to assume that God is aware of the “possible future of the person about to be created”.
The question “Why doesn’t God just not create anyone who would choose to be eternally separated from him?” might seem plausible in the abstract, but we don’t live in the abstract. We live in a world of people, with a plethora of interconnected relationships and interactions which affect our decisions. Salvation is a process, not a one time event. It’s all about what we make ourselves to be oriented toward by the end of our lives. That process doesn’t take place in a vacuum. We are individuals, yes, but we exist within a larger society. Even a hermit exists within and is aware of the society around him, no matter how little he permits it to impact his life. The mere fact that it exists means it has at least some impact.
Take that abstract statement and make it about people rather than abstract numbers. For one, people don’t magically pop into existence. Each and every human person is a body/soul unity, brought into existence in a cooperative act of creation between a man, a woman, and God. The matter for the body is provided by the man and woman. The immortal soul is created directly by God. God creates our immortal soul only when our parents first create the body. If God deliberately withholds the immortal soul from an already created body, do you honestly think there are zero repercussions?
That aside, let’s just consider it from the standpoint of interconnected relationships. What if a particular couple would normally remain faithful at the end of their lives and die with hearts oriented toward charity, but all the children they conceive will turn from God and die without charity in their hearts. If God actively denies that couple children, and one or both of them lose their faith because they are unable to have children and end up dying apart from God because of that, does that mean God must now not create them as well? If he passively permits all involved to make their own choices, then any guilt for wrong choices belongs to the person making the choice. If he actively intervenes and even one person has their choice made, not freely, but as a consequences of God’s direct action, then God has now assumed responsibility for that person’s choice. They would be in hell not because they chose it freely, but because God manipulated the situation such that they chose it.
Let’s consider the potential grandchildren of this couple as well. If any of the grandchildren are destined for heaven if God permits their hellbound parents to be created, then by refusing to create their parents, God has actively NOT created a person who would be in heaven. He has denied the existence of someone who would freely choose to spend eternity in heaven, simply because you don’t want him to make someone who will freely choose to spend eternity in hell. If that person spent eternity in hell because of God’s direct action, you would have a point. But to deny existence and eternal bliss to one being because of another being’s freely-chosen action?
Let’s consider two brothers, one of whom will die faithful and one of whom will die in unrepentant sin. What if the ONLY thing leading the faithful brother to remain faithful is his sinful brother? What if he needs the challenge of his brother to remain strong in his faith, and without that challenge he will fall into sin himself? If God prevents his brother from existing, then God has not passively permitted one brother to choose heaven and one brother to choose hell, but actively decided that the now only brother will be in hell. Does God now have to not create him as well?
If God begins unraveling the interconnected threads of human relationships and pruning out those destined for hell instead of letting things unfold as we will it, will there be any left who are destined for heaven? We meddle in things we do not fully understand at our peril. Assuming that changing A, B, and C will not have negative consequences for J, K, and L, when there’s no possible way for us to know that with our limited knowledge, is the height of hubris. Perhaps the instances of God intervening in human affairs are so rare because it’s only rarely that He is able to do so without directly sending a person to hell. Perhaps we shouldn’t blithely assume that, because we wish it to be so, God can tinker with things over here and cause no negative effect on things over there. Do we really have the all-encompassing perspective necessary to make such a statement with authority?
All of that aside, Christ answers the very question you ask in Matthew 13:24-30 with the parable of wheat and tares:
He proposed another parable to them. “The kingdom of heaven may be likened to a man who sowed good seed in his field. While everyone was asleep his enemy came and sowed weeds all through the wheat, and then went off. When the crop grew and bore fruit, the weeds appeared as well. The slaves of the householder came to him and said, ‘Master, did you not sow good seed in your field? Where have the weeds come from?’ He answered, ‘An enemy has done this.’ His slaves said to him, ‘Do you want us to go and pull them up?’ He replied, ‘No, if you pull up the weeds you might uproot the wheat along with them. Let them grow together until harvest; then at harvest time I will say to the harvesters, “First collect the weeds and tie them in bundles for burning; but gather the wheat into my barn.”’”
So there you have it. The best of intentions can bring about the worst of results.