Are you suggesting we should ignore an ecological disaster because the perpetrators are "just doing their job"?

Citing the historical reasons for their existence doesn't really help the case that they aren't a problem now because the exact reason that it's now and not then. Cats may have had a purpose when they were introduced but in most cases they don't actually serve that purpose anymore. And we can ignore that point entirely because feral cats don't even fit the definition of animals with a job to begin with. They aren't owned by anyone, they are just another animal in the environment. One that happens to be an invasive species causing extraordinary damage to the environment.