The world is going to end, regardless of whether or not it is destroyed beforehand, when the sun goes nova in 5 billion years or so and burns away the entire solar system, leaving nothing but a thin cloud of nebula dust and a slight background radiation. Before this, an asteroid could wipe out all life, a rouge planet could come in and blow Terra to smithereens, a rouge star could float in and pull the entire system out of whack, a black hole could move past Terra and do whatever it does to things that get sucked in, humans could manage to start a space-time ripple and destroy the entire universe, aliens could blow the planet apart to make an intergalactic backway, some god or another could decide to end Terra, and even if it survives all that and the sun blowing up, the universe itself will eventually pop like an overfilled balloon. So why bother worrying about the inevitable (everything ends) when you can worry about things like, say, your fish.