TNVE Trap, Neuter, Vaccinate for rabies, and Enclose them

This is an effective alternative to TNR or trap and euthanize. It is one that avoids both the euthanasia and unethical aspects of enabling to kill.
Letting feral cats loose in the environment even with TNR is not environmentally or humanely responsible. If proponents of ferals cats feel they must protect and maintain the existence of feral cats, they need to be directly responsible for them and eliminate their impact upon wildlife. Instead of TNR, I promote TNVE. Proponents of feral cats can provide giant cat pens like those in a zoo, and set them up on their properties, or in an area close to and with the knowledge of humane societies. Feral cats should not not be released loose into nature. Instead, Trap, Neuter, Vaccinate for rabies, and Enclose them. Release should apply only to a contained Enclosure where they can be managed healthfully and humanely in an Enclosed environment, without destroying wildlife.

Where feral cats are diseased and seriously injured, and / or if there is no feral cat enclosed compound available where they can be maintained in a healthful existence where they can be fed and sheltered without destruction to native wildlife and without introducing an unnatural food source for wildlife, then I do believe painless euthanasia of feral cats is more humane, to both the cats and to wildlife, than letting them loose again.