In any case, a short, happy life in a shelter is probably better than a short, painful life in the wild being eaten by something bigger than you. In fact, the average life span of a feral cat is less than two years on their own according to this, while the life span of a house cat is about fifteen years.
Similar is true of dogs that live without human care. If these wild dogs don’t die of sheer starvation… diseases such as parvovirus, heartworm, or intestinal parasites usually kill them. Their average life span is one to two years. This is confirmed by studies cited in Stafford’s The Welfare of Dogs.
And there are the other factors, as stated here:
“Wherever cats are present they have immense impact on wildlife by
preying on small mammals, birds, reptiles and amphibians, threatening the genetic
pool of wildcat species through hybridization and acting as reservoirs in the
transmission of numerous disease, creating a health hazard effecting both wildlife and
human populations.”
In addition, “dogs, because they are social animals, will usually find other stray dogs and form a group or pack. These dogs quickly forget their human families in the effort to survive. Rural areas are sometimes plagued by groups of feral dogs killing livestock and frightening people.” Source. A survey by the National Agricultural Statistics Service in 1999 found that feral dogs were partly responsible for killing cows, sheep, and goats worth about U.S. 37 million dollars.
So setting your pet “free” is doing no one a good turn.