Origen, go back to being biblical scholar, your eschatology suck.
On more serious note, eternal punishment is just not because it deals with temporal consequences. It's just because it deals with eternal consequences. For in sin, we kill our soul, for grace is life, and mortal sin, sin that kills, cannot exist alongside grace. For what in common does Christ have with Belial? What does Light with Darkness?
Etenral consequences of sin is that you are separated from God. And when you die, your will is absolutly free from time and places. But this means it's fixed. If you die while willingly seprating thyself from God, He, in his Justice and Mercy, will accept your free choice. He left all of Adam to our counsel after all.
But here is the deal that many seem not to see. God is Love. God is Light. God is Life. God is Truth. God is Goodness itself, perfect in every aspect of Good. And unrepented separte themselves from this Good. So it's logically fallow that they are separted from eternal Glory eternally.
Lastly, let me use Aquinas words:
Sin incurs a debt of punishment through disturbing an order. But the effect remains so long as the cause remains. Wherefore so long as the disturbance of the order remains the debt of punishment must needs remain also. Now disturbance of an order is sometimes reparable, sometimes irreparable: because a defect which destroys the principle is irreparable, whereas if the principle be saved, defects can be repaired by virtue of that principle. For instance, if the principle of sight be destroyed, sight cannot be restored except by Divine power; whereas, if the principle of sight be preserved, while there arise certain impediments to the use of sight, these can be remedied by nature or by art. Now in every order there is a principle whereby one takes part in that order. Consequently if a sin destroys the principle of the order whereby man's will is subject to God, the disorder will be such as to be considered in itself, irreparable, although it is possible to repair it by the power of God. Now the principle of this order is the last end, to which man adheres by charity. Therefore whatever sins turn man away from God, so as to destroy charity, considered in themselves, incur a debt of eternal punishment.
Punishment is proportionate to sin. Now sin comprises two things. First, there is the turning away from the immutable good, which is infinite, wherefore, in this respect, sin is infinite. Secondly, there is the inordinate turning to mutable good. In this respect sin is finite, both because the mutable good itself is finite, and because the movement of turning towards it is finite, since the acts of a creature cannot be infinite. Accordingly, in so far as sin consists in turning away from something, its corresponding punishment is the "pain of loss," which also is infinite, because it is the loss of the infinite good, i.e. God. But in so far as sin turns inordinately to something, its corresponding punishment is the "pain of sense," which is also finite.