i know you're saying this from the point of view of your beliefs, and i am in no way ridiculing them. and of course i'm speaking from the POV of my beliefs but...the earth IS nothing but a lump of rock, or as i like to relate it--a grain of sand in the sahara. the earth is not here for our benefit, it's just here. and we do need to take some care of it to preserve our way of life, but there are limits (we're not there for the benefit of the planet). as you say if we eliminate everything that we enjoy we have already ceased to live.
oh and how do we define sentience? i can say that only i am sentient because only i know what goes on in my world, and you're just pixels on a screen (or a figment of my imagination) many other animals have shown what we consider signs of sentience. maybe true sentience is the ability to recognize sentience in others.
if the greenies are so concerned about the environment, why do they all drive cars? how come they don't seem to make sacrifices?