Why are disbelievers punished with eternity in Hell, even though this worldly life is fleeting?

This answer addresses the question of the eternity of Hellfire in three ways:

1. All monotheistic religions believe in Paradise and Hell, and the eternity of both.

2. If the disbelievers were to return to the world, they would revert to disbelief.

3. A person's state in this world determines their state in the Hereafter. There is a realm for pure good (Paradise) and a realm for pure evil (Hell), and neither of these will perish. There is also a third realm for those who possess both evil and good (for disobedient monotheists), and this realm will eventually cease to exist, as the disobedient will emerge from Hell after receiving their punishment.