Interesting, but I disagree with your conclusion.
First of all, travel outside of the earth was inconceivable just 150 years ago. So why discount the idea the FTL will not be possible in the future.
Secondly while your thought on being visited by robots seems valid, that they have the mission simplicity of a sperm seems counter intuitive to me. If they have the desire to explore space, they most likely have all the other aspects of intelligence. And if they are that advanced, then they should not have the desire to knock us off. We would not be completing with them, so why kill the ants?