I took on some outside ferals after my agency had adopted out the kittens. I live in the country, and have a fairly safe area for outside cats. But one kitty (Captain Feathersword) disappeared pretty quick. Another (Peaches) made friends with a stray who turned up after I adopted the 3. I trapped the new stray and got him neutered. He was so wild that the vet even commented on how feral he was. I released him immediately after he was recovered from the neuter. But somehow Peaches got injured...I found her the day after her buddy got neutered with severe facial injuries. Maybe something went wrong with the trap, and she got injured. (I had 2 traps out.) Or maybe having her buddy gone and unable to protect her left her open to a raccoon or other predator. I did see her one more time, I think. At least it was a kitty with her coloring, and it seemed to be fine. The 3rd cat, Jeff, hung around for at least a year. And he and Captain Feathersword were brothers, and looked pretty similar. The kids would have occasional Captain sightings, but I don't know if it was him or his brother Jeff.
I believe they are all gone now. Whether dead, or moved to another area. We have lots of cats near here, as it is farm country.
Long story...but several people at my rescue think it would have been more humane to euthanize the 3 ferals. I'm glad they had a chance, and some time to live away from the feared people. And due to Peaches, I was able to trap and neuter her buddy.
And if you TNR in the place of origin, I think you have more success with keeping the kitties. They are already part of a colony.
Ideally, someone would build a couple of huge enclosures, so the ferals can live safely. But that is a huge responsibility. And without TNR, the outside cats will never quit reproducing, and there will never be enough enclosures.
I am a strong advocate of TNR. One of my current foster kittens will go back outside to where he came from if he does not tame enough to be adopted. He has a safe place to go, and reality is that there are more kittens and cats than homes to place them.
Edited to add: My shy foster went to his first adoption show today, and did just fine. He will not be going back outside, but is destined to be a housecat. I knew he had tamed some, but today he showed that he could tolerate a lot more people than I thought he would. Compared to his hissy brother in the next cage, who spit at anyone who got too close, Wit looked positively tame! (Yay!)