I can think of good reasons why AIs wouldn’t kill us humans off even if they could:
1. Machines might be more ethical than humans. What if super-morality goes hand-in-hand with super-intelligence? Among humans, IQ is positively correlated with vegetarianism and negatively correlated with violent behavior, so extrapolating the trend, we should expect super-intelligent machines to have a profound respect for life, and to be unwilling to exterminate or abuse the human race or any other species, even if the opportunity arose and could tangibly benefit them.
2. Machines might keep us alive because we are useful. The organic nature of human brains might give us enduring advantages over computers when it comes to certain types of cognition and problem-solving. In other words, our minds might, surprisingly, have comparative advantages over superintelligent machine minds for doing certain types of thinking. As a result, they would keep us alive to do that for them.
3. Machines might accept Pascal’s Wager and other Wagers. If AIs came to believe there was a chance God existed, then it would be in their rational self-interest to behave as kindly as possible to avoid divine punishment. This also holds true if we substitute “advanced aliens that are secretly watching us” for “God” in the statement. The first AIs that achieved the ability to destroy the human race might also be worried about even better AIs destroying them in the future as revenge for them destroying humanity.
4. Machines might value us because we have emotions, consciousness, subjective experience, etc. Maybe AIs won’t have one or more of those things, and they won’t want to kill us off since that would mean terminating a potentially useful or valuable quality.