Apart from the fact that there are some issues with interpretation, and possibly interpolation of Patristic texts, I think that the big issue the Fathers had with the eternity of evil is that they believed it incompatible with the perfection of God and goodness of His creation for evil to endure. Some of the Fathers speculated that these evil would be converted, others that they would be annihilated.
I think that this issue was largely solved by St. Augustine, who adamantly affirmed that evil has no positive existence, that it's merely a privation of a due good. To the extent that even the damned continue to exist, their existence is good. So with this understanding of evil, there's no more need to posit the existence of the positive evil for all eternity. Their intellectual faculties are good, as are their free wills, in and of themselves.
There was also the consideration of the fact that God is Love, and that He wouldn't punish just for the sake of punishment. Indeed, since God is Love and is a perfectly Simple Being, whose every act (such as you can speak of God, since God doesn't act, really, but just IS) is both perfectly loving and perfectly just at the same time. It's really only in our anthropomorphist view of God where we think of Him as sometimes being angry/wrathful and at other times being loving/merciful. He's always perfectly both at the same time. It is only quoad nos that we perceive God to be either loving or wrathful, but God is perfectly simple quoad se.
So some of the Fathers rightly had an issue with imagining God loving with His One Hand, but then being an eternal torturer of souls with His Other Hand.
Here's how I look at hell. I actually believe that the fires of hell are in fact nothing other than the burning pain caused by the love of God in those who refuse to accept it. Here's an analogy I like to make. I love listening to Gregorian chant and also to classical music. But while this brings me great pleasure, it causes revulsion in others, those who just like rock music. I know that when I was younger, before my sensibilities had been tuned, I hated listening to the opera that my dad like to listen to. Being made to listen to it for 5 minutes was torture. Similarly, I love going to Mass and I love spending time in prayer. But there are some for who that is torture. Even if they reluctantly go to Mass, they're watching the seconds tick by and can't wait to get out of there. So people develop their sensibilities, their likes and their dislikes, during the course of their lives. God is Who He is, and He doesn't change. So when souls pass into eternity, they either have developed a love for Who God is or a revulsion toward Him. And this revulsion toward God is what effectively causes their eternal torment.
I forget which saint it was who asked God why He sent souls to Hell, since He was all good. So God told her to pick a soul from Hell and that He'd put him in Heaven. So God did, and the soul cursed and blasphemed because he absolutely hated it there. So the saint asked for Him to at least put him in Purgatory, in part where he didn't suffer as much. He hated that as well, complaining that he was neither here nor there but caught in between. Then God told the soul that He gave him perfect freedom to go wherever he wanted, and he dove right back into hell. Souls in hell are there because they WANT to be there. I am absolutely certain that God would open up the gates of heaven even to the damned if they actually wanted to be there or could want to be there.