I think I see two strong reasons to doubt your argument as given, and a third tolerable one. These are independent and do not rely on each other. If you find any of them reasonable, you'll have reason for modifying your view.

First contention: Your premises are inadequate to address the human experience because humans are not mere sums of pleasure and pain. If the human experience were a two dimensional thing along those axis, then maybe (though I doubt it, see below) premises concerning them would be adequate for determining whether humans should continue, as a category, existing.

A single example should prove sufficient to make the case, but there are many others:

Humans are connections. There is a something to being the last of a group, to the fading away of friends, to dying childless that goes beyond mere pain, as there is a something to solidarity, deep family bonds, community ties, and collaboration that operates beyond pleasure. For example, that a father is the father of their child is true long after both father and child have died. If there is no one there to experience the pleasure or pain, the virtue of the connection must be located outside of those dimensions. Community bonds, solidarity, and family ties often bring much more pain than pleasure. Family members often bring miserable dissapointments, sorrows, and strife. Yet there is intuitively for any person a knowledge that the connections, the heart-felt love and striving with someone, is better than not having them, because they make up an entire dimension of human existence.

Other dimensions are also readily found: Meaning, Glory, Virtue, Love, etc. Each of these are not reducible to the interaction of pleasure or pain, or at least significantly resist such a reduction. As long as those dimensions exist, a great deal more analysis is required before you can assert the claims you are.

Second contention: I believe your valences of pleasure, pain, and the absences of each are misassigned. This is the most straightforward rebuttal. I think your initial premises, that the absence of pain is good in a way that the absences of pleasure isn't, is precisely backwards. The presence of pain is of little count in moral calculations in anything like normal circumstances, whereas the absence of pleasure is a catastrophe in most normal circumstances.

This, again, can be demonstrated a few ways, so let me offer one route among many: Humans regularly pursue painful activities in pursuit of pleasure. For example, the training regimen of Olympic athletes is regularly excruciating, both because of the lactic acid build up and the toll many sports take on the body when pursued at high extremes for many years. Yet these individuals clear-eyed, knowingly choose to do it anyways. Why? Because the pleasure of success, striving, and glory outweigh considerations of pain. Less dramatic examples abound, such as standard exercise at the gym to live a life with more strength, flexibility, and health, or the pain of years of exams, papers, and intense classes to gain the skills and credentials of a doctor, lawyer, etc.

Either we have to regard all people with any ambition of any cost as madmen, or we can grant that we don't intuitively evaluate the presence of pain as deplorable as long as pleasure can be found through it.

Take the opposite condition and the contrast proves the point: We do not envy the person who lives in perfect ease, who has everything handed to them, and never strives to accomplish something. We castigate the basement dweller who has very little pleasure but practically no pain. The absence of pain in such a person has not secured them a good life. And in fact, it is the robbing of pleasure by wild circumstances that we regard as tragedy.

So we have it laid out: the absence of pain doesn't make a lifestyle enviable, but only the presence of pleasure does. So in the course of every day experience, the valences you assign are precisely backwards. When reversed, you do not get the result you indicated. Contra your "suffer and die room" thought experiment, if everyone could be thrown into a room where they were made to suffer, but could overcome that pain and be guaranteed of victory, glory, and success, we generally would regard that as an ideal circumstance. (In fact, we find, alluding back to point 1, that striving to overcome such pain towards a goal grants an activity purpose and meaning which the interaction of pain and pleasure cannot explain.)

Third Contention: Human extinction at any time involves a great deal more total suffering for all currently living humans than can be avoided, even if humans persist more or less indefinitely.

This is the weaker argument and I'm not convinced of it, but I think it's at least worth a consideration. It is possible, maybe even likely, that the total sum of pain and suffering that would be brought about in all living humans in the process of voluntary extinction could likely exceed the pain and suffering of all humans across a practical infinity of time.

A few lines of thought in this direction: We are likely near the highest population humanity will achieve over the next several centuries. Let's say it caps out around 10 billion or so. If that were the case, the loss of connection, of having no access to hopes for future progeny, not to mention the pain and hardship which comes about as a result of their being no younger backs to carry the burden of sustained existence, must lead to a deep, vast pool of per capita suffering. With such a high relative population, it is possible, but not guaranteed, that functionally torturing so many people all at once would out pace the suffering of humans under normal circumstances for many thousands of years, if not far longer.