For not all who are granted a renewed existence by the resurrection will enter upon the same new life. Rather will there be a great difference between those who are purified and those who lack purification. Those who in their lifetime here have already been purified by baptism will be restored to a state akin to this. Now purity is closely related to freedom from passion, and it is not to be doubted that blessedness consists in this freedom from passion. But those, on the other hand, who have become inured to passion, and to whom nothing has been applied to cleanse the stain -- neither the sacramental water nor the invocation of divine power, nor the amendment of repentance -- must necessarily find their appropriate place. Now just as the appropriate place for debased gold is the furnace, so the evil mingled with these natures must be melted away in order that, after long ages, they may be restored to God in their purity. Since, then, both fire and water have a capacity to cleanse, those who have washed off the stain of sin in the sacramental water do not need the other means of purification. But those who have not been initiated into this purification must of necessity be purified by fire (Gregory of Nyssa, Address on Religious Instruction, 35).
It is fascinating and compelling to me that Gregory uses the metaphor of gold being purified to describe sinners who go off into hell: they are creatures of great value, but they have been debase and need to be cleansed. This is why their time in hell cannot be infinite: because they are of value, the appropriate course of action is to purify, not to destroy, just as it would not be appropriate to get rid of gold if it were defiled. Presumably, moreover, their value comes from the fact that they are creatures of God, made by him in his image (or alternatively, that they contain, as might a fetus, the potentiality to exemplify his image). This fact -- that even despicable sinners are of infinite value -- is often forgotten or disregarded by many who argue against universalism.
In an article of his combating Thomas Talbott ("In Defence of Divine Retribution," I think) Oliver Crisp makes a statement to the effect that he sees no reason to think punishment enacted strictly on grounds of retributive justice with no hope or possibility of reconciliation and restoration is a "second best" to the eventual realization of restoration and reconciliation. In other words, he doesn't see why it would always be preferable to restore and reconcile if possible. The reasoning, however, is actually perfectly clear: people are of considerable value independently of their actual moral worth, and for this reason it is always better to restore them than not.
Consider an alternative scenario: is it not obviously preferable to restore, as opposed to scrapping, the classic car given to you by your grandfather as a birthday gift once it has broken down? So long as the car is of value to you and repairing is within your means, it's obvious that it would preferable to restore rather than to scrap.
So also in this case: people are gold, people are of tremendous value even in spite of their sinfulness and wretchedness, because (say) they are creatures of God made in his image. (If you denied this, say, in the case of the reprobate, you would be hard pressed to find some rational motivation for condemning the murder of a reprobate person.) And if God is God, then he not only knows what is valuable but also values it. And of course it is within God's means to save all persons. Therefore this is what is right for him to do, this is what it is appropriate and just for him to do. The alternative would be inappropriate: it would not be appropriate treatment of gold to toss it when it is defiled; likewise it would not be appropriate treatment of a person who is of infinite value to destroy her (or allow her to destroy herself).