I am disturbed at the absolutely horrific things that some humans go through. The very worst things I can think of include child sex trafficking and the fact that young children are sometimes raped and abused by family members, including their parents. I have read stories about the torture of children by psychopaths. The suffering these children must go through must be unimaginable to those that have not experienced it.
I was thinking about sharing specific details of the most disturbing acts I have read about but decided that may be inappropriate, even though I think sharing specific details of atrocities may strengthen my argument. If anyone’s interested, read the Wikipedia page of serial killer Albert Fish (not for the faint of heart).
My point is that preventing human extinction inevitably subjects many, many more children to these atrocities. This doesn’t sit at all well with me and I don’t think it should sit well with any reasonable person.
I suspect the main comeback to this is that as humanity improves we will eventually see a day where these atrocities don’t occur. I think this is just way too optimistic. Even if this is achieved it could be millenia before we completely eradicate all abuse. I doubt that millions more abused children is a price worth paying.
I’m not saying we should encourage extinction, I’m saying we should cease efforts to prevent it. We should redirect these resources to making the world a better place, not prolonging its existence.
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.
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.
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.
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.