Having worked for an animal shelter that served a large area that included a few urban cities, a lot of suburbs, and was located in a rural area, I would never dream of letting my cats outdoors. We got multiple calls a day that would bring tears to your eyes because of what happened to someone's outdoor or indoor/outdoor pets.
Between catnapping for revenge (happened on a monthly basis), shootings, dog maulings, dissapearances, stolen cats, poisonings, hit-by-cars, cats being fed on by predators, sickness and a whole host of other unbelievable things - it's not a safe world for a cat. In the past hundred years, cats have had to deal with exploding human populations who might not care to have them in their yard, a whole lot more dogs, predators that have adapted to catch "easy" domesticated prey, new and deadly diseases from a serious over-population problem, and worst of all an ever-enlarging road network with road-raged drivers who AIM for cats - I've seen it happen!
Not only that, but most shelters are so overrun by the feral and stray cat population, so most shelters will kill cats within hours of them arriving - there is no mandatory holding period for cats like there are for dogs. When a cat goes missing, owners usually wait a week or more to report it. In that time period, a busy shelter may have seen 150 tabby cats that look just like the description (and photo!) of lost "Fluffy", so most owners can't even know for certain that their animal was euthanized, and are left wondering forever what may have happened. And the ownership laws for cats are not like for dogs - cats are owned by whoever's property they are on - meaning that if they are in your neighbors yard, that neighbor can take the cat anywhere they want and legally nothing can be done to that person. In some states it is legal to shoot a cat that is on your property as well. (Thankfully, where I live is NOT one of them, but I've never seen anyone prosecuted for it and the cat still ends up dead.)
I've been told that the average life span for an outdoor cat is 2 to 3 years. This would include the average lifespan all feral, wild, stray, and owned cats who live part or all of their lives outdoors, and includes all unwanted kittens. (In the US, every single person in the country would have to own 9 cats for every cat to get a home. That means a family of four would have to have 36 cats.) I'm not sure how true this is, but when people came in looking for a new outdoor cat, it's because the old one had been killed, and the majority of these were 2 years and under.
There are some cats that can survive outside, and I can understand a person wanting to do that. However, I think the safety of the cat must be weighed just as well as the happiness of a cat. I always believe in safety over everything - which for me means indoors only - but I have never turned someone away who realized that it's dangerous and takes precautions to make sure the right cat has a fighting chance to live a long life. I do not believe in bringing home a 2 month old kitten and letting it outside. I also don't believe in doing that with an older cat that has been indoors all of it's life. It's almost always instant death, expecially in winter! I think that any cat who goes outside needs to a be teen because it has an idea of what life is like, and is capable of learning quickly. I also know there are cats that age who could not make it - mainly those who were born and lived inside all of their lives. And those who are stupid. I've met many many stupid cats who have thankfully been fixed and so would not be passing their genetic material to the next generation - because normally they wouldn't have been able to survive to do so. I have own lot of these myself.
Now, I've met some people who have had outdoor cats live to 14 years old- but they've been far from major roads and don't have many neighbors, and the cats have been born and raised by an outdoor mother. I think the education from the mother is the most important part, and I think that outdoor cats should stay with their mothers until they reach 6 to 8 months so they have a great starting point.
However, I think the best evidence for indoors is how many indoor only cats are living to great ages and are still happy and active. We see more people every year who come in with stories of their 23 year old cat, and even older! I had one woman who was so adamant about me believing that her cat was 26 years old that I thought she was lying and got suspicious. I did a veterinary reccomendation call, and lo and behold, the cat was seriously 26 - documented and everything!
I have 22 cats of my own now, thanks to working for a shelter and having a fiance who did the same. They are all strictly indoors. They have three floors to roam on - two having a completely open floor plan - 1800 square feet plus the basement. There are 18 litterboxes, 5 food bowls, and 5 water bowls. They range in age from 14 years to 8 months, and I still have trouble thinking that they aren't all 8 months because they all play the same! It's a lot like keeping fish - there is a saturation point when you get too many in a small space, and there is "water quality issues" when it comes to litterboxes and sleep areas, and compatability issues between individuals. But in a clean house with a lot of attention and extra litterboxes overstocking is only noticed by guests.
Okay, so I have to sweep every other day, and the litterboxes get scooped twice a day, but I love having guests over because the cats all come by to say high, and the guests eyes just keep getting bigger and their mouths keep dropping farther open as more cats arrive, and then comes the remark I love to hear "How many cats do you have? I can't smell anything! I have only one and all you can smell is cat at my house!"
It's a life choice. 4 pounds of food a day (Iams multicat has worked wonders on waistlines!), 20 pounds of litter a week (Tidy cats blue scoopable - I swear by it becuase I've tried everything else to much odor). 22 visits to the vet a year plus emergencies. It's given me a great perspective on cat ownership, and just how happy cats are in large groups, and finding that there's not much that can't be dealt with with just a little more effort. (Yes, behavioral problems like peeing can be corrected, and so can fighting.) Lots of cleaning, lots of grooming, lots of hairballs. Lots of getting woken up in the middle of the night by talking, howling, fighting, and little dingle balls I never should have bought. Always having a cat on you, or next too you, or stretching on you. Lots of little holes in the bottom of every shirt from the boys hanging on to ask to be picked up.
I guess the real verification I have that they are alright inside is that only one tries to get outside - I could leave the door propped open and none of them would care. The one that does is the 8 month old kitten who isn't afraid of anything - be it strangers, or loud noises, or the vaccum, so I think it's just her spirit to go exploring. (She'd probably get eaten by the neighbor's dog trying to make friends with it though.) (Okay, and to be honest the largest cat who is a little over a year old tried to get out at Halloween because he wanted to greet all the kids as they came to the porch - I plan on leash training him for next year though!) The cats will even pass over the open windowsill to curl up on the couch with me, so obviously they're content!
And yes, there are days upon days when I ask myself why didn't I get 22 hamsters instead, or bettas, or cory cats, or sheep, or llamas, or frogs....... and then one of the kitties comes along, flops into my lap and starts purring, and I end up in bed with 10 little buggers all sending me their "I'm happy and now you are, too" vibes.