One of the problems of annihilation for atheists and secular humanists (and Seventh-day Adventists and Jehovah's [False] Witnesses) is that it is unjust - if both Hitler and a little old lady who never harmed anyone both face the same fate after death, how is that just? (If you then try to add a purgatorial period of suffering that lasts relative to ones crimes against humanity, then you again run into the logical issue of time in eternity). Another problem is that it removes a large justification for morality - if we all suffer the same fate (annihilation), then eat, drink, and be merry no matter how much that causes others to suffer.
The better option for atheists and secular humanists would seem to be a system of reincarnation where, when the soul separates from the body at death, the soul does not leave time but is reinserted into a new body of a greater or lesser lifeform (based on the relative merit of ones previous life) until one lives such a good life that they enter eternal bliss - it has the concept of purgatory without the logical issue of time, it's based on the annihilation of imperfection rather than existence, and it has all the attraction of other works-righteousness systems. But, like annihilationism, it is incompatible with the testimony of Scripture:
"If a man die, shall he live again? All the days of mine appointed time will I wait, till my changing shall come." (Job 14.14)
"And as it is appointed unto men that they shall once die, and after that
cometh the judgment: So Christ was once offered..." (Hebrews 9:27)
Annihilationism amongst Christians is an attempt to make God seem more fair, but part of the doctrine of Hell is that each gets only and exactly what they deserve: "And that servant that knew his master’s will, and prepared not himself, neither did according to his will, shall be beaten with many
stripes. But he that knew it not, and yet did commit things worthy of stripes, shall be beaten with few
stripes: for unto whomsoever much is given, of him shall be much required, and to whom men much commit, the more of him will they ask." (Luke 12.47-48) This part of why the WCF states that everyone will "receive according to what they have done in the body, whether good or evil." (33.1) So I believe that not everyone in Hell will be suffering to the same degree - they will all suffer according to the degree in which they knew the truth and rejected it.
...there certainly is an eternal awareness of isolation and alienation from God...
I believe this is true to the extent of being isolated and alienated from God's love, but I think the most terrifying aspect of Hell is that God, being omnipresent, will be there "for the manifestation of the glory... of his justice" (WCF 33.2).