So, what about the cleanup effort? Let's say you're a civilization advanced enough to direct world-destroying missiles at everything 100 LY around you... that

So, what about the cleanup effort? Let's say you're a civilization advanced enough to direct world-destroying missiles at everything 100 LY around you... that means you can reasonably get some intelligence on what you're destroying, and it's probably valuable enough territory to make it worth checking out before you incinerate it. A 100-year mission to go there and, oh, alter the atmosphere just enough to eradicate the dominant species, would be a much better strategy than just annihilating a productive ecosystem...