Sorry this post got kinda long...TLDR is: the fear is that we might already be in the 'best of all possible worlds' and there is no escape.
I think the core problem is the plausibility is hell is very high even for rational people.
Think of the stereotype (I think I've heard of studies that show it's not true or something?) about intelligent or ruminative people being pessimistic and negative. "Being realistic is being negative" line of thinking.
Once you are awakened to how miserable life is, it seems very hell-like already. The basic observation that suffering greatly outquatifies joy. And joy always has a cost or consequence. So much so that even survival drive seems cruel. Good experiences are apportioned to the most minimal degree possible to keep you struggling (its own form of suffering). The memories of very good experiences are stripped of intensity such that it takes deep meditation and optimism to milk much joy out of them--they instead serve as another carrot on a stick to keep you fighting. Meanwhile, negative experiences are haunting and hyper-real and can readily bring back nearly the same intensity of agony, tears, and trauma.
Many people love nature and think of it as relaxing. Things like the "Planet Earth" documentaries are popular. They play that series in a room with massage chairs at my local gym. When I go in there to 'relax' I am treated to scenes of animals tearing each other apart, losing babies, starving as they trot across expanses of foreboding terrain, and the like. It's haunting. Hell at every level.
When we are shown 'awe-inspiring' nature like massive waterfalls, cosmic features, and so on, that awe is just mesmerizing because we are beholding raw power and unforgiving nature. Awe is adjacent to fear. We cannot usually relaxingly ride a massive waterfall or encounter massive cosmic bodies safely. It's the human fantasy and dream that we conquer nature through space exploration, technology to explore deep oceans, and all that. A fantasy that we are not powerless, that our will can overcome and support an existence of joy. Fantasy. Hope to keep us struggling.
Because there is no control. Ultimately, likely no freedom or will as we would wish.
Sorry if this seems tangential but there is a point.
We are talking about nature itself being a fractal of suffering. The very state of being is horrific.
Hell seems plausible because:
1. We cannot deny that we are here now. "I think therefore I am" is horrifying.
2. We observe that matter and energy are not readily created or destroyed. We know that the universe contains unimaginable amounts of both and understand little except the terrifying timescales these things persist.
3. We know absolutely shit about shit. We are aware that we are limited in our senses, knowledge, and processing power. We need technology to even perceive the full range of the electromagnetic spectrum just for a quick example. Our eyes are limited to a tiny range. This ignorance cripples our understand on far more than just vision: what else are we not aware of, and struggle to slowly build technologies to even begin to touch on? Looking at the cosmic scale, we don't even know what the fuck really happens with black holes. Even if we figure that out, there is always another mystery around the corner.
4. "Death is just like before we were born, just think--what do you remember? So there's nothing to fear." Memory wipe makes this moot. We are born into a new form/organism. A lot of 'enlightened gurus' harp on and on about all is one, the self is illusion and all that. Not much of a comfort. See point 2 about the unimaginable quantity of matter, energy, and timescales. And these are just things we can observe, which as in point 3 we inherently know shit-all. That we are here at all (point 1) is already a 'what the fuck this shouldn't be happening' so rationality is not much a comfort that we are safe from suffering at the end of this particular torture phase.
So I agree that point 4 IS powerful. We can take some comfort in knowing that death at the very least ends this particular storyline. And even if we are reborn, it'll be right back to ground 0--memory wipe.
This is kind of true to our everyday life though. We "die" every moment because our attention is limited and we are only experiencing a small window at a time. This is our conscious experience. It is linked together by our brains into this story of self and memories which make it feel like we are persisting as a coherently with seamless continuity. We go to sleep each night yet there we are every morning to suffer anew.
Imagining Sisyphus smiling is not much of a comfort for me at least. Just try zenning out the agony of people ripping you apart slowly with pliers.
So it is not out of the question that some form of awareness will persist and it won't be pretty. Life itself is all about 'organisms' organized matter and energy competing for control of matter and energy. Most of the joy of life comes from harming other organisms. Eating is the prime example. It wouldn't surprise me if photosynthesis or the joy of viewing a sunset is hurting some conscious being somewhere. In a way it is. Because you are taking matter and energy that was presumably unaware, therefore not in pain, and incorporating it into an organism, which is a state of struggle.
So here's to the joy of at least hopefully the permanent death of the current organism in which I am instantiated. A temporary reprieve at least. Possibly more hell to come.