Even with TNR or family owned cats killing the animals in the area...
The cats are also spreading diseases amongst themselves and other animals in the area as well. So by reducing the number of feral cats you also reduce the spread of feline HIV, FIP, panleukopenia, micoplasm, feline herpes and feline Chlamydia. And then there's cat scratch fever, toxoplasmosis and ringworm that tend to plague cats as well.
It sucks, but overall the general population of felines will be better off as well as the environment. There's a cat colony I know of and can't help anymore because it's too out of hand and none of the rescues or animal control want to work on it. They're so sad and mangy looking now... I own a cat from that colony when I rescued some abandoned kittens from them.
He was diagnosed with herpes, mycoplasma, Chlamydia and may have some sort of liver condition. Thankfully not hiv/leuk/fip but still quite a bit for a cat that we took in at 3 weeks old. @_@