Nah, I'm just not under the illusion that we are special in any way. The universe doesn't give a shit about whether we survive or not, that we often want to is simply a personal bias, not an inherent truth, and those personal biases can vary..
Whether we survive or not is just a thing that happens, it's not inherently "right" or "wrong". No matter what happens to us the universe keeps turning, doing it's thing.
Perhaps the goal of preventing human extinction is the one arbitrary priority, the one "unicorn in the garden" [orain.org], as a starting point to make atheism practical.
You don't need that assumption to make atheism practical. Atheism is practical because "god" is not a falsifiable concept, this doesn't make it wrong, but it puts it in the realm of other things of that realm like the tooth fairy and santa. Things that even if they so happened by some freaky coincidence to be right there is no possible way to have evidence of it because of the lack of ability of the conjecture to be tested.
Critical rationalism allows atheism to function. Not in a hard "there is no god, for sure" stance, but in the "there is no reason, and can be no reason to prefer the conjecture that there is a god, so I'm going to totally ignore it like all other untestable things" sense.