On the other hand, there are also arguments for why one should work to prevent extinction even if one did have the kind of suffering-focused view that you're arguing for; see e.g. this article. To briefly summarize some of its points:

If humanity doesn't go extinct, then it will eventually colonize space; if we don't colonize space, it may eventually be colonized by an alien species with even more cruelty than us.

Whether alternative civilizations would be more or less compassionate or cooperative than humans, we can only guess. We may however assume that our reflected preferences depend on some aspects of being human, such as human culture or the biological structure of the human brain[48]. Thus, our reflected preferences likely overlap more with a (post-)human civilization than alternative civilizations. As future agents will have powerful tools to shape the world according to their preferences, we should prefer (post-)human space colonization over space colonization by an alternative civilization.

A specific extinction risk is the creation of unaligned AI, which might first destroy humanity and then go on to colonize space; if it lacked empathy, it might create a civilization where none of the agents cared about the suffering of others, causing vastly more suffering to exist.

Space colonization by an AI might include (among other things of value/disvalue to us) the creation of many digital minds for instrumental purposes. If the AI is only driven by values orthogonal to ours, it would likely not care about the welfare of those digital minds. Whether we should expect space colonization by a human-made, misaligned AI to be morally worse than space colonization by future agents with (post-)human values has been discussed extensively elsewhere. Briefly, nearly all moral views would most likely rather have human value-inspired space colonization than space colonization by AI with arbitrary values, giving extra reason to work on AI alignment especially for future pessimists.

Trying to prevent extinction also helps avoid global catastrophic risks (GCRs); GCRs could set social progress back, causing much more violence and other kinds of suffering than we have today.

Global catastrophe here refers to a scenario of hundreds of millions of human deaths and resulting societal collapse. Many potential causes of human extinction, like a large scale epidemic, nuclear war, or runaway climate change, are far more likely to lead to a global catastrophe than to complete extinction. Thus, many efforts to reduce the risk of human extinction also reduce global catastrophic risk. In the following, we argue that this effect adds substantially to the EV of efforts to reduce extinction risk, even from the very-long term perspective of this article. This doesn’t hold for efforts to reduce risks that, like risks from misaligned AGI, are more likely to lead to complete extinction than to a global catastrophe. [...]

Can we expect the “new” value system emerging after a global catastrophe to be robustly worse than our current value system? While this issue is debated[60], Nick Beckstead gives a strand of arguments suggesting the “new” values would in expectation be worse. Compared to the rest of human history, we currently seem to be on a unusually promising trajectory of social progress. What exactly would happen if this period was interrupted by a global catastrophe is a difficult question, and any answer will involve many judgements calls about the contingency and convergence of human values. However, as we hardly understand the driving factors behind the current period of social progress, we cannot be confident it would recommence if interrupted by a global catastrophe. Thus, if one sees the current trajectory as broadly positive, one should expect this value to be partially lost if a global catastrophe occurs.

Efforts to reduce extinction risk often promote coordination, peace and stability, which can be useful for reducing the kinds of atrocities that you're talking about.

Taken together, efforts to reduce extinction risk also promote a more coordinated, peaceful and stable global society. Future agents in such a society will probably make wiser and more careful decisions, reducing the risk of unexpected negative trajectory changes in general. Safe development of AI will specifically depend on these factors. Therefore, efforts to reduce extinction risk may also steer the world away from some of the worst non-extinction outcomes, which likely involve war, violence and arms races.

My rough answer to this is: If someone wants to die (after thinking about it for a long time and having time to reflect on it), let them die. If they want to live, help them do that. The vast majority of people want to continue living. I don't see how the atrocities that are experienced by humans outweigh the benefits, given that the vast majority of humans seem to have a pretty decent will to live.

(This does not hold for animals, and I think the strongest arguments for antinatalism and promoting extinction come from considering non-human suffering, but that seems different from the case you are making)