It's one thing to say that there are too many people on Earth and we need to stop breeding irresponsibly or we'll all suffer. That's an accurate statement. It's quite another to claim that the extinction of the human race, voluntary or otherwise, would be a good thing. Good for who? There'd be nobody left. Yeah they talk about the "Biosphere" and the "Environment," but those things only have value so long as there's somebody around to enjoy them. A field of clover with nobody to look at it will bring zero joy to the world.
The extinction of humanity would mean the end of poetry, film, politics, music, literature...half the things that make this world wonderful. It would also mean that there's nobody around to appreciate anything else. Who cares if the coral reefs rebound if the only entities that experience them are eels?
The environment is a resource. Like any renewable resource it should be taken care of, lest it become destroyed, but it's not good in and of itself. Likewise the irresponsible actions of humanity don't mean it's not the greatest species living on earth and worthy of preservation. The fact is that the reason we need to control our behaviors is not because we're REALLY worried about some beetles going extinct, it's because we're fucking ourselves over by heating the planet and destroying important resources. Yes there should be fewer humans on the Earth. Significantly fewer. But that's not the same as having none as a goal.
Plus if every rational person stopped breeding then who would be left creating kids? The morons and the fundies. Already the fact that third-worlders breed at rates far beyond the West and developed East is causing problems, because you have vast populations of uneducated underemployed people cooped up in smallish countries, threatening to flow into and capsize our...I'll say it...better societies. And yes, assimilation works, but if you have 50 million people entering a society of 25 million...who will end up assimilating?
Anyone who wants humanity to die out is creepy and wrong. What we need is for humanity to live smarter and more responsibly.