Isn't the flip side of this that human pleasure is good, and if there are no humans to please, there is no human pleasure? Why do only negative sensations matter?

Suffering is more intense than pleasure. A unit of suffering and a unit of pleasure at the same time will result in a net suffering effect. Starvation or torture is much, much more intense than even the highest pleasure a human is physiologically capable of, like orgasm. The possible pleasures are not worth the certain sufferings.

Also, I don't only oppose reproduction because all humans suffer and to give birth is to condemn a being which did not previously exist to certain suffering. I oppose it also because that being can't consent to being born, for obvious reasons. No human is deserving of the right to pluck a being out of eternal nonexistence just for it to suffer and then plunge back into that nonexistence again.

Only workable and possible if people actually WORK to provide it.

No, because as I've said, automatons and AI could do the work to provide it, it being the luxury of not having to work at all.

So shouldn't we be throwing all our available funds at the development of such an AI? Shouldn't we tolerate human suffering now for the promise of eventually eliminating it later?

Human suffering will never be eliminated. Suffering is ineradicable, inescapable, in a being. But we should strive with all of our available resources to minimize it, yes, and the way appears to be the development of AI and automatons. Of course, the end of reproduction would be a better solution. It's literally the best solution, in terms of the reduction of human suffering. But people don't see that, and won't see that. Regardless, the fact remains that the surest, most effective, and only way to eradicate human suffering is for there to be no humans to suffer.