I agree. Being alive means you're gonna use some energy, and you're gonna pollute a certain amount.
To my way of thinking, unbridled population growth is the biggest environmental threat. If population is allowed to grow unchecked, in the long run, no amount of conservation/clean energy/etc. will prevent ecological degradation.
We're talking about a declining quality of life, and not necessarily human extinction.
I heard an interesting speaker at the Commonwealth Club a year or two ago explain that limits on the amount of potable water on the planet will impose a limit on human population later in this century. For poor countries in Africa and south Asia (including India) and many other places, getting clean drinking water will become harder and harder. The incredible increases in lifespan that those places have seen over the last 75 years—due in large part to fertilizers and modern agricultural methods—will begin to be lost. People will die younger. This speaker predicted a leveling out of the population at around 9 billion. That ain't extinction, but 10 to 20 years off your life certainly qualifies a reduction in its quality.
In the first world, you may not even notice that the quality of life has declined. More and more, for instance, we now have to eat salmon that are grown in the pens of "fish farms." Do you think a farmed salmon—one that swims back and forth in its pen—tastes the same as a wild salmon that roam the oceans freely? I don't. Many of our children and grandchildren, however, will never have tasted wild salmon. They won't even know that their quality of life has diminished, because they won't know what they are missing.