What threat would there be? Sounds irrational since it's logical that if aliens needed precious resources, our universe is overly abundant with them. They could harvest platinum, gold..etc from many other lifeless worlds. Water is abundant in the form of ice, it's everywhere...well everything is everywhere. Any Type 2 civilization or higher has no need for biological slaves or any of the such.
Furthermore, if any aliens that could make it here want to destroy us, all they have to do is hurl an asteroid of a minimal size with the right trajectory from anywhere in our solar system or beyond and with enough velocity and gravity calculations, we would never know they were there or have time to respond. That's all it takes, no need for war ships or them risking their lives, just an energy supply capable of harvesting and throwing a giant rock at us. Send some robots in the wake of aftermath to harvest whatever....but i'd say it's still way easier to just mine a lifeless planet.