But say the pessimist is wrong about all this. Say the bleak yet honest perspective of the pessimist is entirely off-center. Say the universe is actually overflowing with goodness, or at least not nearly as bad as the pessimist makes it out to be.
Where does that place the pessimist?
If the universe really is such a great, fantastic place to be, how could the pessimist be so wrong in their evaluation? Essentially, this would require the pessimist to be under some sort of illusion - and yet, illusions are precisely what the pessimist is focused on removing. The pessimist explicitly believes that there are illusions at play.
The same cannot be said about the optimist, who cannot believe that illusions are at play on pain of contradiction. If the optimist is wrong, then their thesis is discarded as naive and wrong - and illusion. Their thesis was an illusion. But the failure of a pessimist's thesis simply vindicates their thesis - that there are illusions, dangerous and misguided illusions. There is a clear asymmetry between optimism and pessimism.
Discarding pessimism without considering the metaphysics of doing so essentially boils down to victim blaming - it is the fault of the pessimist for being so misguided, when in reality it is the fault of the universe itself for having the ability to produce and sustain such brutally dark yet misguided ideas. The existence of negating-pessimism is incompatible with any affirmative ideology. A truly holistic affirmative ideology requires the non-existence of illusions, especially such apparently-misguided illusions as pessimism.
Any sort of fascist state requires obedience, servitude and assimilation. Fascism is built around fear - fear of the unknown, fear of the unnatural, fear of the other. This ideology places a rift between the state and the barbarians, the good and the evil. It is, fundamentally, a dualism that ignores any sort of unifying, holistic picture of the world. It cuts Being down the middle, separating what is seen as good as entirely ontologically different from what is evil. It builds walls around the good to protect from the evil, without recognizing that this is an act of cosmic self-mutilation, i.e. an act of war between the universe and itself as it forgets the common heritage between its parts. Like an arm fighting its opposite without realizing they're connected to the same body.
This is what any sort of optimistic, affirmative ideology ends up being: a quasi-fascist attempt to keep out dissent, as if contrary opinions were a strange alien to the landscape and not fundamentally derivative from the landscape itself.
And we can see this quite easily when looking at, for example, a PTSD patient. PTSD patients have to learn to "move on" from their past, come to terms with it and heal their wounds. While an honest, yet painful, perspective is one that recognizes the problem and doesn't pretend it's not there. It's one that knows that any sort of shift in perspective is going to require a sequence of forgetting what happened.
That is why pessimism is virtually undeniable: the very existence of pessimism confirms its thesis.
The only alternatives that I can see require theological convictions, such as the belief that all evil is simply the absence of God. This places the good outside of the world, transcending the bad. This is exactly what see in certain Biblical books, such as Ecclesiastes, which tell us that life cannot deliver what we seek and that we must turn our heads to the transcendental, to God. But this is hardly "optimistic". If anything it's just another flavor of pessimism - it rejects the world as a decaying slab of mediocrity for an idealistic realm of possibility. The difference here is in the existence of hope, which is sustained by yet another dualism - a split between the world and God, the cave and the World of Forms, dukkha and nirvana, etc. It's a philosophy that dresses up escape as redemption and hope, one that accepts everything the pessimist has said but anchors hope in the other-wordly as a last-ditch effort for comfort.