I'm not smart enough to dismiss cautions that AGI will lead to human extinction, but many of the arguments I have seen, including Bostrom's, don't adequately explain that AIs are not life, and therefore do not compete for resource the way life does. That is, an AGI will not be alive, but neither can it die. It doesn't have genes, and it will not have evolved to propagate genes competitively.
So, while it's possible that AGI will be as bad or worse than competing with a more powerful and intelligent species of animal, they're not animals, and probably won't behave like animals. Increasing chimp intelligence, on the other hand, is probably not a good idea. Bonobos, maybe. Not chimps.
Also missing from the discussion is that intelligence isn't the only danger from competitors for resources. Insects collectively weigh a multiple of the total human biomass and many of them would eat us all, given half a chance. Microbes could evolve to kill us all, directly, or by modifying the ecosystem.