Typically, we find out what niche a critter had filled after the fact, when we see the results of eradication.
The Laws of Murphy and of Unintended Consequences tend to teach unpleasant lessons.
Meanwhile, snakes kill dogs. My in-laws lost a dachsund to a rattler a few years back. They could just as easily have lost the dog to a red-tailed hawk. Should we hunt hawks to eradication?
Emotional, irrational responses to threats are how we normally like to define antis. Obviously, those responses are not limited to antis...