Extinction would prevent future generations being born into this world that is so full of suffering.
Given the level of human suffering on earth, the extinction of the human race would be a blessing. It would prevent future suffering inflicted on future generations.
Human sentience opens the human race up to suffering. Life is so gruelling, painful and full of suffering that the kindest thing to do is not have children and allow the human race to become extinct.  In this sense, reproducing is an intrinsically cruel deed. It subjects the new life to the horrors of this world. Far from a tragedy, the extinction of the human race would be the kindest, most compassionate end to mankind and sentient life.
Almost everyone, when asked, would say they are happy to be alive. Even those in extreme poverty, the severely disabled, and the mentally ill take great joy in being alive.  This is reflected in the behaviour of many people. There are endless avenues for suicide all around us (tall buildings, noxious car fumes, rope) yet, comparatively speaking, very few people commit suicide. This is because almost everyone believes it is better to exist than not to exist.
P1: Life is gruelling, difficult and fraught with suffering. P2: To be born into this life is bad. P3: Therefore, it is far kinder not to subject future generations to this world. P4: Therefore, to allow humankind to die out would be an act of kindness and compassion towards future generations.
Rejecting P2: To be born to this life is not bad.