Two of the verses sometimes used to argue for the position that hell is eternal state:
Verily I say unto thee, Thou shalt by no means come out thence, till thou hast paid the uttermost farthing.
And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him.
Thinking about this long ago, I had assumed that the man would eventually be able to pay off the debt, even if it took millenia to do it. So, I was confused about how this would be a good illustration of eternity, unless the point was that millenia is an analogy for eternity.
But what I had neglected to consider was basic economics. The man owes the massive debt in today's money. But a talent of gold tomorrow is worth less than a talent of gold today - and a talent of gold in a hundred years is even less. There is time value to money.
The amount of money owed in the Matthew 18 parable is massive. Some have suggested that a talent corresponds to about 6,000 days wages. Thus, 10,000 talents corresponds to about 60,000,000 days wages. If money had no time value, it would take 164,000 years (or so) to pay off the debt. But if the time value of money is even 1/1000th of a percent, the man would never be able to pay the interest on the debt - each day he would owe even more than the previous day.
P.S. I provided a clarifying comment in the comment box, but perhaps it makes sense to place it in the main post:
It's not a question of inflation. Even in an inflation-free system, a dollar (or ruble, or yen, or euro, or peso, or pound) today is worth more to people than a dollar (or whatever) a year from now. That is to say, even if the dollar will buy the same sandwich in one year that it buys today, we'd rather have the money now, thanks. It's a fundamental principle of economics.
And Matthew, Matthew's audience, and (obviously) Jesus were aware of the concept of interest. Interest factors explicitly into another parable (the parable of the talents, see Matthew 25). In that parable, the lord was not satisfied to merely have his talent back at the end of the time period. He expected that it should at least have received interest.