In a perfect world, residents would own their cats, all felines would be sterilized, and no animal would ever be abandoned. Yet, not living in a perfect world, efficient and effective solutions must be drafted that minimize the impact of feral cats. Cats are opportunistic killers. Given the choice between scavenging or being predators, they will choose to be scavengers - but will continue to kill wildlife. So the question of impact is moot.
Cats do have an impact on wildlife.
The real question is whether this impact is sustainable. Research has shown that in non-island environments feral cat colony impact on bird life is believed to be sustainable. Where this impact is not sustainable is in areas where endangered wildlife may be present, or on closed environments such as islands. Studies have shown that bird predation by cats is less than their predation of rodents, insects, and reptiles. The premise that bird populations are being threatened to extinction by cats is not well supported.