You are significantly underestimating what is possible. For one we are talking about a civilization millions of years older than us (very likely, the chance of

You are significantly underestimating what is possible. For one we are talking about a civilization millions of years older than us (very likely, the chance of intelligences evolving within a few thousand, let alone a few hundred, years of each other is negligible.) They will likely have hit physical limits of what is possible technologically long ago. Maxxed out tech trees, essentially. It's possible they can have dyson spheres, and artificial brains the size of planets, running far faster and more efficient computers and algorithms. This is of course just wild speculation, I'm only trying to give a sense of how absurdly far ahead of us they could be. Interstellar travel is difficult yes, but possible. We've had the basic technology requirements for interstellar spaceship since the 50's (project Orion.) If you accept the premise that it's possible to send a reasonable size package (possibly as small as a pin if nanotechnology is possible) then wiping out a primitive planet doesn't seem so difficult. >If the civilization was that scared of being wiped out, it'd probably be cheaper to build a single defense for their civilization rather than a bunch of attack platforms intended to wipe out other civilizations. Possibly. It depends entirely on the costs of defense against advance civilizations vs the cost of wiping out primitive planets before they become a threat. We don't know what futuristic interstellar warfare would be like, but my guess is crushing intelligences before they advance to far, is far easier.