1. I was wondering if we could apply an anthropic type of argument here? We are having this argument at this time and place because we predate the colonization by self-replicating bots / other end-of-world scenarios. Multiverse version: any time we're having this discussion, we are on the surviving branch predating self-replicating bot colonization that renders the planet unlivable. I realize this type of argument can be extended to reach absurd conclusions, but nonetheless it's an entertaining thought.
2. What is life? What is civilization? What are their goals? Why do we assume a society of relatively independent individuals? Is it even a sustainable model for a post-human level civilization? Aren't we anthropomorphizing aliens a bit too much? What if a civilization either dies out at human-level stage or necessarily reaches a mental unification point and all its further activity is inward focused? I think we're on the first stages of self-awareness compared to rocks and jellyfish, imagine the level of self-awareness experienced by a far more advanced civilization. Would it necessarily be resource and space-greedy and try to colonize the galaxy? At least it's not immediately obvious to me why it would.
3. Another point is, what scales are we looking for? Doesn't our scale essentially depend on the planet / star system we evolved in, and the scale at which original life started? What basic building blocks / resources does alien life use? Maybe they rely on physical resources we haven't yet explored (dark matter, etc). Essentially I'm agreeing with the people saying "we don't know what we're looking for".