The predators will never disappear in the first place.
The fewer predators there are, the more wasteful defense mechanisms are, and therefore the more disfavored they are by natural selection. The more predators there are, the more defense mechanisms are favored. The equilibrium is neither zero defenses nor 100% effective defenses against predation. It's somewhere in-between.
The predators get to evolve too, by the way. There's no 100% effective defense against all forms of predation.
The fewer predators there are, the more wasteful defense mechanisms are, and therefore the more disfavored they are by natural selection. The more predators there are, the more defense mechanisms are favored. The equilibrium is neither zero defenses nor 100% effective defenses against predation. It's somewhere in-between.
The predators get to evolve too, by the way. There's no 100% effective defense against all forms of predation.