Immortality in freedom is impossible for those who are not submitted to God. Can God in his omniscience and omnipotence allow that which in anti-God, that which is evil, to be and still be God? I don't think so, it would be saying that he is not sovereign minimally, and ultimately would make evil intrinsic to him. For evil to remain in what he's created would mean that it was reflective of him, that it is one of his attributes.
Granted, some Universalists envision the flames of hell as producing the most beneficial change (repentance) in the those being toasted--God as the ultimate Skinnerian, I suppose; an evangelical approach to purgatory, perhaps. Not only is there not so much as a peep about such a conception in the scriptures, the Apocalypse seems to render the difference between those inside the New Jerusalem and those outside with finality. We have this life until we die to change our minds about God, through the Gospel and the conviction of the Holy Spirit we have our opportunity.