To claim it's not a good thing to eliminate the possibility of more future people is to claim there is a moral obligation to produce children. I could be mistaken, but this seems like a reasonable if/then claim. It's not a claim I make, as I don't claim it's not a good thing to eliminate the possibility of more future people.

BTW, what would people here think of the following argument? If refraining from having children is good/rational/moral/[insert applause light here], then good/rational/moral/[insert applause light here] will do so; bad/irrational/immoral/[insert boo light here] will have more children than them, and (to the extent that badness is inheritable -- not just genetically but also memetically) in the next generation there will be a larger fraction of bad people. That doesn't sound like a good outcome; good actions predictably leading (causally and/or acausally) to bad outcomes means that your ethics system is broken; therefore, the assumption that refraining from having children is good must be wrong.

(I tentatively endorse it, but not with very much confidence.)