But life is not only suffering, there are good parts in life too

Not if you are a "farm animal". Not for hundreds of billions of sentient beings whom their lives is one consecutive horrible experience. For hundreds of billions of sentient beings there are no good parts in life, only fear, pain and suffering. From birth to death. That’s life for hundreds of billions of sentient beings bred into in this cruel world every single year.

Imagine a situation in which over 90% of the people in the world are physically deformed, suffering from constant pain most of their life time. A global nightmare.
Do you think that a statement such as that life has good parts too would be made if that world were the reality? Well not only that it is, our real world is much worse than the one you have just imagined. This is an everyday reality for a population which is 7.5 times larger than the human one, annually! This is the reality of 90% of the chickens raised for meat.

The lives of farm animals are so absent of good parts that by far the luckiest "farm animals" on earth are male chicks in the egg industry.
Unable to lay eggs and not genetically manipulated for profitable meat production, the males in the egg industry are killed as soon as they hatch.
That is how hellish this world really is if you look at life from all earth’s beings’ point of view.

If you object the idea because life has its good parts too, we ask you to observe things from the point of view of one spermatozoon. What are the chances of the newborn baby to be happy?
Most humans think about life and about happiness from a human perspective only.
Well the chances of the one spermatozoon to be a happy human are not very good.
To state the obvious, there is a 50% chance it will be a female. Of course it doesn’t mean she can’t be happy, but it means she will be automatically and systematically discriminated against for her entire life just because of her gender. Just one example out of many, merely for her gender belonging, there is a 25% chance that she will experience some sort of sexual abuse.
The spermatozoon has 12% chance to be white. 6% chance to be a white male. And less than 3% chance to be a white male in a western country.
It has more than 50% chances to be very poor. Obviously we don’t think that poor, nonwhites from non-rich countries can’t be happy, but they definitely have worse starting points.
The spermatozoon has a 20% chance to live with lack of safe drinking water and 30% chance to live without water for basic hygiene.
6% chance it will be a slave.
6% chance it will suffer a mental or behavioral disorder.
18% chance it will suffer from hunger.
25% chance it will live in dangerous, unstable situations.
20% chance it will be illiterate.
30% chance it will be in a constant risk of getting malaria.
40% chance it will be at risk from dengue.

The list is practically endless but the point is clear.
There are so many suffering causes and that is when the spermatozoon in our little thought experiment turns out to be an individual from the most privileged species on earth. Things get significantly worse when we calculate the spermatozoon chances to be happy if it turns to be ANY being in the world.
The spermatozoon has 14,000 times the chance to become an individual from a commercially exploited species than to become a human, based on a course of one year only (including estimations of directly consumed marine animals, by catch, and fishmeal).

The math is very simple. A non-speciesist perspective, necessarily leads to the conclusion that when considering every sentient being, life is definitely mostly suffering and there are almost no good parts too.

We think that ethics must solely focus on the bad experiences of the victims and not weigh them against the good experiences of the ones benefiting from their victimization. It may sound trivial but classic utilitarianism for example weighs both sides of the equation, some schools even in an equal manner.
But not all of them. Negative utilitarianism for example doesn’t consider good experiences as morally relevant. And so do we. We find negative utilitarianism as by far the most ethically relevant moral philosophy. But even if you don’t, the call to annihilate the human race does not deprive anyone of good experiences anyway. If no one exists, no one is harmed by the fact that potential good experiences are not fulfilled. The dead are not experiencing anything, including not the deprivation of good experiences. So they will not be harmed by their death. Death is bad for the living who grieve and miss the dead. But the dead don’t feel anything anymore, therefore can’t be harmed. We realize it might be counter intuitive for some of you (while stating the obvious to others), but death is not bad for the dead. It can be good for them if they had suffered while living, or neutral if they enjoyed their lives, but it can’t be bad since ’the nonexistent’ can’t be harmed by the negation of good experiences. The dead are dead. They cease to exist. They can’t be harmed at all. It’s not that they are moved to an observation room where they can watch what they are missing by dying. The dead don’t mourn the experiences they were deprived of after they die. The nonexistent don’t feel anything anymore, they can’t be pleased or harmed.
Don’t confuse death with dying, thinking about dying, near death experiences, or living under death threats. These are all by no doubt harmful experiences, but they are of living individuals regarding death. Humans can definitely suffer from death but only as long as they are alive.

So if the dead are not harmed by missing the good parts of life, and 150 billions of sentient beings a year have no good parts in their lives, how is human extinction not the moral solution?

After humans are gone, no one will experience a life full of suffering so others can have good parts, and no one will suffer from their absence.

And finally, consider the following question - what kind of a world do you prefer?
A world in which there is not even one suffering sentient being who is born to a life of systematical exploitation and suffering from birth to death (not to mention trillions of which), and a much smaller number of beings that life and pleasure were prevented from them but they are not hurt by that since they were never even born (the never born are never hurt). Or a world in which billions of beings are daily tortured so that a much smaller number of beings is able to enjoy the good parts of life?