One of the biggest arguments for indoor cats is that they live longer. I do wonder to what extent this varies by region, and particularly rural versus urban, since only one of my cats' deaths could have been prevented by me keeping him indoors, and only two could've been prevented by them being indoors their entire lives. (Hermes probably already had FeLV, an infectious disease he likely got from other cats outdoors, when I got him.) Most of my cats died of age-related conditions, and the only other premature deaths we've had were from kidney disease in two brothers, which was almost certainly genetic. (One is questionably premature since he was 10 years old - in human terms he'd have been in his sixties.)
Recently, I found a news article about a research study (unfortunately the paper doesn't seem to be publically available) in which they affixed cameras to outdoor-roaming cats' collars in Georgia, US, to see where they went. The study is interesting, but I take issue with their idea that the data they found indicates that you should keep your cats indoor. Specifically, I take issue with their list of 'dangerous activities' the cats engaged in while outdoors.
Crossing roadways, shown by 45% of cats - I'm surprised it's not higher. In most human settlements, you can't really get anywhere without crossing roadways. But although many cats do die from being hit by cars, cats don't seem to be that much worse off than humans in that regard. I've witnessed many cats crossing roadways myself, both mine and others. Typically they will stop, look both ways, and then if no car is coming they will flatten themselves down and run across. Whereas I've known several dogs who had no clue about road safety, most cats are pretty cautious about cars. Cars are more a danger for wild creatures than for domestic ones, because wild animals are less likely to know how cars move. Domestic cats understand that cars stay on roads and that they move much faster than any animal does.
Eating and drinking things they found, done by 25% of cats - well, they don't say what the cats were eating and drinking. It's important to keep in mind, however, that cats have a keen sense of smell, and generally prefer the smell and taste of things that are healthy to them. (Incidentally, the idea that cats like the taste of antifreeze is probably a myth; at the very least dogs don't. They may drink it if they're desperate, however.) Humans actually are too cleanliness obsessed, it's been shown that our concern with cleanliness is contributing to our rates of asthma and allergies. If you drank from a mud puddle, the risk of it harming you is minimal.
Exploring storm drains, done by 20% - well, this is a danger, but only in the case of a serious storm. Many outdoor cats will head indoors when a storm comes. If not (eg if they're too far from home), I suspect they would hear the water rushing and realize the storm drain is not a safe place to hide from the storm, and look for another hiding spot. Again, cats do understand that high ground is better for a storm.
Entering crawl spaces where they could become trapped, done by 20% - OK, this one is just ridiculous. Cats know if they can fit in a small space, because their whiskers reach out the same width as their body. If they are considering entering a hiding spot, they'll feel it out with their whiskers first. In addition, there's plenty of small spaces indoors that a cat could potentially get trapped in. About the only way a typical cat would get trapped is if their exit was blocked off after they entered.
Although the danger seems to be overblown, I do agree that indoor cats probably do, on average, live slightly longer. But at what cost to their quality of life? If you confined a human in the house, set up a safe environment where they can't hurt themselves with anything, and kept them there, they'd live longer too. But their life wouldn't be as enjoyable. And human homes are designed to be enjoyable to humans - both cats and dogs have made it clear to me that they don't find being indoors as enjoyable for them as it is for me. In particular, the smells inside a house are far less interesting than the smells outdoors.
I really feel that most animals, like humans, need a stimulating environment - especially when they're young. They need to have room to explore, to experience new sensations, to interact with other members of their species (with a few exceptions, regarding animals more solitary in nature than cats are) and generally broaden their horizons. In the case of dogs, you can meet that need by regular walks, but cats are pretty hard to walk. With small caged pets, simply taking them out of the cage for supervised play gives them this stimulation, but an entire house is similar in relative size for a cat to the better cages for smaller pets.
I'm not saying that an indoor cat will always be an unhappy cat. Many of my older cats essentially became indoor cats by choice when they got too tired and sedate for outdoor play. And in a multi-cat household, with humans who often play with the cats, they can get pretty good stimulation. But it's a lot harder to make an indoor cat's life be a good quality one.
And I do think there is some good in taking risks in life, in order to enjoy your life completely. I trust my (adult) cats' judgement about their own safety. (I do supervise younger kittens on their early trips outdoors, because they're less safety-conscious. But as they get older, I let them roam more freely.)