> An alien civilization that can traverse the enormous gulf between solar systems can very easily build widely spaced telescopes that can monitor all nearby sol

> An alien civilization that can traverse the enormous gulf between solar systems can very easily build widely spaced telescopes that can monitor all nearby solar systems for changes to atmospheric composition. Maybe they can't themselves, but don't like the idea that someone else eventually may be able to. I think you're overconfident of the safety of actively advertising our presence to the universe because you're making a lot of assumptions and only considering a small number of possibilities. The problem is that there are so many unknowns and so many shades between different possibilities that no one can really be confident about anything, except that the risk could be anywhere between zero and the maximum.