Good topic. Here are some of my thoughts.
Reflecting on these topics can bring up different emotions, none of which is necessarily more truthful than others. Calling the idea of nibbana as mere cessation “inherently pessimistic” and “undeniably dark” I think universalizes a personal value judgement: not everybody will see it as such. Many may find it dark, indeed, but many will not. For me, when I first heard of it I was literally laughing out loud out of appreciation. I found it a very relieving and unique idea, especially compared to other religions. I saw a certain beauty in finishing a long journey without needing any outcome, and a certain wisdom when the Buddha bowed down to a truth higher than his own desires.
It all depends on our frame of reference. I think I can show by analogy how this is so.
Many people might wish for an eternal heaven where they meet their deceased relatives, listen to heavenly music, fly over fields of butterflies, and what not. But most Buddhist, including those who see nibbana as a type of existence, would reply that such experiences are impermanent and still suffering, so that going beyond them is not a dark thing. They just shifted the line on what they consider to be suffering beyond such heavens. Now, if we just move that line even further, including the more subtle kinds of existence, we can conclude that the end of existence isn’t necessarily a dark thing either. It depends on our perspective.
In other words, Buddhism is indeed inherently pessimistic: pessimistic about samsara, that is. But some Buddhist (including the founder, I would say!) simply include all forms of existence in samsara. Does that make them pessimistic while others are optimistic? I don’t think that is the right way to frame the debate.
But it’s true, on a bad day even stream winners, though having right view, when they aren’t reflecting properly, may prefer eternal existence in samsara over nibbana. Cessation may seem like a somewhat empty goal. However, for them such thoughts will be fleeting. Since deep down they have understood the Dhamma, their disillusion (nibbida) with existence will prevail. It is no longer about what is “preferable” but about what is truth. The truth is that there is nothing worth holding on to, so they stop holding on. Like the Buddha, they bow down to truth and set aside their desires. Also, a vision of the truth includes the realization of rebirth, so ideas of suicide to end one’s one and only life will not come up.
When I have taught on this topic, I made sure to add a disclaimer: if these ideas make my audience feel very uncomfortable or otherwise unstable, it is probably not a good time to reflect on them. There is such a thing as proper attention (yoniso manasikara). Proper attention is not just about what you pay attention to, but also at what time you do it. The best time to do it is after a deep meditation, because then the hindrances, including fear and discontent, are gone. That’s largely why the suttas say samadhi (which includes joy and bliss) leads to seeing things as they really are, not pondering them in a state of sadness and worry.
In fact, I even would go so far to say that drawing certain conclusions about this when the mind is not ready can block samadhi, which thereby block access to right view and all progress. I think this is why even senior monastics get stuck in certain eternalist views. The kind of letting go you need to do to enter jhana is similar to letting go of all ideas of all permanent existence. But if you’ve already concluded that consciousness “that is what you are”–to quote a prominent western ajahn (and the Upanishads…)–you won’t be able to do that kind of relinquishment.
So the best way to approach it, in my view, is to not preemptively decide what is pessimistic and what is not. Instead, develop other factors of the path such as virtue and meditation until the mind is ready.