Here's on of those times I disagree with Frame - I think this is the best of all possible worlds given God's goals. Obviously the world isn't perfect, at least not in the sense that it's without taint, without flaw, incapable of evil. In that sense, I'd agree with Frame. Who wouldn't? But, it is perfect in the sense that it perfectly accomplishes it's telos - it is the perfect means to the perfect end, namely, God's glory.
Why do you argue that it is merely possible that this is the best of all possible worlds?
People sometimes say that God must make the best possible world because he himself is perfect. So they think that although evil exists now, this is nevertheless the best world God could have made. That is one traditional attempt to solve the problem of evil.
I disagree, however. Genesis 1:31 says that God made everything good, but not perfect. “Perfect” would mean not only good, but also incapable of becoming evil. Clearly God did not choose to make that kind of world. In that sense, the new Heavens and the new Earth (Rev. 21:1) will be a better world than this one, for that world will be confirmed in goodness, incapable of becoming evil. So the world in which we presently live is not the best possible world. God is free to make a world that is imperfect in some respects.
Could God have made a better world than this one? Certainly. He could have made what we call the “new Heavens and new Earth” right back at the beginning. Why, then, did he choose not to do so? I don’t know. That is essentially the problem of evil. I think there are some biblical ways of addressing the problem, but I don’t think we will have a completely satisfying resolution of the problem during our present life.
It certainly has been tainted by our sin, but doesn't our sin have a role to play in God's overarching plan to see his glory maximized? Certainly. Without sin there would be no need for grace, mercy, compassion and hence no appreciation of them. Without sin there would be no need for redemption, no need for a slain lamb, who was, after all, slain before the foundation of the world. Without sin we would not be witnesses to the awesome wisdom of God as displayed on the cross. (Please note, I'm not at all intending to call sin good. Best the read this in light of Joseph's words to his brothers - we meant it for evil, God meant it for good. Also look to Paul in Romans 6:1-4 & Romans 9:19-24). Read again the wonderful verses of Eph. 1:7-10:
"In him we have redemption through his blood, the forgiveness of sins, in accordance with the riches of God's grace that he lavished on us. With all wisdom and understanding, he made known to us the mystery of his will according to his good pleasure, which he purposed in Christ, to be put into effect when the times reach their fulfillment —to bring unity to all things in heaven and on earth under Christ."
Given that God is a wise God, an omnipotent God, and a good God, it seems inevitable to me that we would affirm this is the best of all possible worlds. Wisdom is, to quote Tozer, that philisophical powerhouse, "Wisdom, among other things, is the ability to devise perfect ends and to achieve those ends by the most perfect means."
Does Scripture directly teach that this is the best of all possible worlds? No. Does it teach it indirectly by asserting God's wisdom, by reminding us that God does what God pleases, by holding up for worship a God who is sovereign and omnipotent? I think it does. Seeing this as the best of all possible worlds certainly requires a shift in viewpoint - we must see it against the backdrop of God's eternal and inviolable purposes. Only then can we see it's ultimate perfection.