IMHO, "supervised" doesn't cut it either. If you think your mere presence is sufficient to protect your cat, you're only fooling yourself. Being within earshot? Great...you might be so lucky as to hear the squealing tires--and the thud.
Jackson tells a story about one of his clients who was outside one morning, standing on his deck, with his cat sitting right next to him. He was drinking his coffee and enjoying the sunrise. Suddenly he heard a funny noise and looked to see what it was. He saw, already a long ways off, a coyote with his cat IN ITS MOUTH--snatched from RIGHT NEXT TO HIS FOOT. The guy yelled, the coyote dropped the cat (who was, thankfully, not injured), and all concerned were definitely sadder, but hopefully wiser.
Coyotes are as fast as greyhounds. Are you?
Jackson (poor guy) also had the misfortune to actually witness a hawk carrying off a screaming cat.
Redtail hawks (average size for a hawk) have a four-and-a-half foot wingspan, dive at 120 miles per hour, and can fly quite a bit faster than you can run--even carrying a cat.
Ever try to stop a catfight? Wouldn't recommend it.
Face it--as a human, you simply do not have the ability to react in time to stop EVERYTHING that could possibly happen to your cat.
Granted, a few cats live long and happy lives outside. My neighbor's outdoor cat was 15. They got a kitten. Sweetest little black kitty you ever saw. Let him out immediately, I guess he was about 10 or 12 weeks old. I complained. They said their older cat would teach him to be street smart. Guess he was a slow learner, because he died right in front of their house, killed by a car on our very busy street long before his first birthday.
My upstairs neighbor's cat, Boots, was sitting on his own porch one summer day, just 2 weeks before his 20th birthday. We had a big party planned for him. He was attacked and torn apart by two pit bulls, who played tug-of-war with his broken body. Unfortunately, he was not killed outright, but was privileged to suffer for a few hours until he was euthanized. Happy Birthday, dear sweet Boots. I miss you so much. I cry every time I think of you.
Think about this: when you have just a handful of cats who reach old age outside, how many other cats have to die very, very young to bring the average age of death down to less than 5?
None of these people wanted or expected these hideous deaths for their beloved cats. But all of this suffering could have been prevented by one simple thing: keep them inside. It's your choice, but it's your cat's life.