I made what you were stating clearer. Why would not saving endangered species lead to all the weakest lifeforms on the planet going extinct? Is it the fate of all of those lifeforms to go extinct without human intervention?
It’s only a matter of time before we kill off what remains and you have to wonder, if we made earth so inhospitable to life, for how long can we survive ourselves?
Specifically, just to hit this point home, the impact of humans is an extinction event comparable to the Cambrian–Ordovician extinction event, or the impact that killed off the dinosaurs. Who(or what)ever exists billions of years from now will be able to view the immensity of our impact assuming they still have the tools we have today.
That's a much better formulation.
It doesn't have to be "obvious that killing all the weakest lifeforms on this planet would cause bad things to happen" not to do it.
It just has to be possible and uncertain.