It's hard for a lot of people to hear, because if you have had outside cats a lot of your life it's very heartbreaking to hear something that seems to say that you abused your animals, especially if you really loved them.
Also a lot of us as kids had outside cats and we didn't have any control over their living conditions, but it still creates much guilt.
That's not the point though. The point is that culture changes and we know more and have a better ability to do right by our creatures, and we can do Our Best now just as we did Our Best in the past.
Many people are not conscious of cats being non-native predators, don't think of them as invasive species because they were never told that. It's fine to not have known something.
It's also fine to not have known that cats can thrive indoors and they don't need to go outside unsupervised to be happy. There are very common myths about how cats can't be trained and how cats don't get enriched by interaction by their owners that dogs do.
With that said—it is a good idea to keep cats inside. I grew up with outdoor cats and had so many of them get gruesome injuries or diseases and die very young. There is the occasional lucky cat that makes it a long time, but most outside cats die well before their time. They get in fights with other cats, they get mauled by dogs, they get hit by cars, they get all sorts of worms and other parasites, and they get poisoned or shot by malicious people who don't want cats on their property. One of my childhood cats, I found under a bush with her guts ripped out. Things like that happen to the cats that "disappear."
These things can happen even to a smart cat. You can't be sure your neighborhood is safe. Your neighbor two doors down might get a Husky tomorrow that mauls cats. A drunk driver might tear through your otherwise quiet neighborhood.
Your cat doesn't have a cell phone that they can call 911 if something is trying to kill them. They can't yell "Help me!" if they are stranded somewhere with a broken back or some horrible injury.
A cat isn't thinking, "I'm confident that I can deal with all the dangers out here!" when it goes outside. Cats roam around because instinct makes them feel that it's a good idea to maintain a big perimeter of territory, not because they necessarily like doing it. They don't know what a disease is. They just catch a disease and they know that they hurt and feel awful, and sometimes they die.
For all these reasons I think the choice that makes sense is to keep your cats inside when you can't supervise them or keep them in an enclosed space. They're animals that don't know what we know, and we have the ability to keep them safe from lots of things. So I think we should.
