I prefer to interpret eis aion as "into eon" instead of "for ever"

Why does the Bible literally claims that hell is for ever. The problem may be with the word "for ever." There is limited reason to interpret this word as one that conveys eternity. The mix-up may be with the word "never." The word "never" is eternal, that is to say, if a person states that they will never do something, that means for all eternity that thing will never happen. But if you ask someone will they 'ever' do something and they respond with a yes, that does not mean they will be doing that thing for all eternity; only for 'ever.' For 'never' is eternal but for 'ever' is not! The word "ever" is connected with the word "event" and "eventually." People who burn in hell will burn for an event, for an age. The words "for ever" are used many times in the Bible to signify events that don't last eternally. Jonah was, according to Jonah 2:6, in the belly of the whale for ever.

The actual word that is translated as 'ever' is the same word that we get the English word "eon" which many times in the Bible is translated as “world.” Which means the word eon has similar connotations as the word "ever." An event can be defined as a scene, a picture, a time, an event is simply something that happens, a reality. Another clue is the word that is translated as “for,” it is translated as "into" in many other places in the Bible. Evil people are burned; into ever, into the world, into ashes, into smoke, into nothingness, into agelessness, into eon. (It is possible that many unclean spirits will burn “day and night”before they are completely turned into nothingness. Revelation 20:10) Remember only God hath immortality, even your soul is mortal. (1 Timothy 6:16,
Matthew 10:28)

It refers to an indefinite period of time. Exactly how long the period of time is, is determined by the subject. So one possible way of translating it is, "to time indefinite." In the instance of the wicked, they will burn until there is nothing left to burn. John's point was evidently to emphasize that the fire won't stop until it's completed its purpose of destroying sin and unrepentant sinners.

The fact that Revelation talks about non-physical beings burning in fire shows that the fire it refers to is not the ordinary kind of fire we see on earth today. It is a fire that comes down from God out of heaven, like the special fire that God sent to destroy even the rocks on Elijah's altar.

But "into eon" sounds cooler.