It goes beyond that. Even if interstellar travel is not plausible, you'd still expect to find solar systems completely transformed by intelligent life (think Dy

It goes beyond that. Even if interstellar travel is not plausible, you'd still expect to find solar systems completely transformed by intelligent life (think Dyson spheres, although maybe not so extreme). If interstellar travel were merely supremely difficult, then you'd expect regional clusters of stars exhibiting these properties. These would be bizarre astronomical oddities that we'd have likely noticed by now.