I have adopted a feral cat. I've also worked with them in the past (adoption preparedness at a no-kill shelter). Bringing them indoors, getting them used to being on a lead and harness outdoors (or otherwise supervised and contained), is one of the kindest thing you can do for them. For the ones living in colonies outdoors and incapable of being re-domesticated (the ones you wouldn't try to adopt in the first place), there's a reason TNR - trap, neuter, return - is a thing.

Cats started associating with us all those thousands of years ago precisely because, generally speaking, they enjoy the comparatively "easy life" of living with ammenities and low stress. Perhaps we should have never encouraged their domestication in the first place; I'd leave that discussion for another day. But we're here now and we have a responsibility.

If the feral cat cannot be socialized and kept in a safe environment where it cannot harm local wildlife populations then it should be euthanized. No amount of useless downvotes will change that fact, or my unwavering position on environmental protection and my pure disdain for people that think it's okay for cat's to roam free outside.