We adopted two kittens late last year, and thought it would be nice to let them outside to run around and explore nature. We used to have an indoor cat that lived in apartments his whole life, and I always felt a bit guilty since he wasn't very happy and would always try to get outside.

Our kittens learned to hunt very quickly and would bring animals into the house. Lots of lizards, mice, and birds. We got tired of burying dead animals and vacuuming up feathers. Then one of our cats got into a fight and got bitten. The vets think that the bite must have punctured a vein or artery in their shoulder, which caused a huge hematoma that swelled up their chest. They had severe anemia and almost died from internal bleeding, so they had to give them a blood transfusion. They didn't have any cat blood available, so they used dog blood instead. (Apparently that's quite common!) He's almost fully recovered now, but still has a limp that is slowly getting better. The vet bills ended up being over $6,000. Fortunately we had pet insurance, so we only had to pay $1,300.

So we don't let them outside anymore. We were surprised to see that they don't seem to mind too much. They're not begging to go outside and they still seem perfectly happy. We've put a little bit of grass on an internal balcony, and we might try to build a catio enclosure so they can still have a little area to go outside. Maybe some netting like this: https://catnets.co.nz

EDIT: Cat tax: https://imgur.com/a/umQpcvK

I think people tend to overestimate the desire of most cats to spend time outdoors, because as humans we ascribe human motivations to cat behaviors. So we see the cat repeatedly try to go through the door, and we think of this as persistent behavior that reflects a high degree of motivation to go "outside". But "outside" is not a concept that a cat really understands. They understand doors well enough, and they might be frustrated and upset by not being allowed to go through them. But, as you mentioned, there's actually a lot more for them to be frustrated, upset, scared, and injured by outside than there is inside.

> "outside" is not a concept that a cat really understands.

This seems like unfounded speculation. Why would they understand doors but not inside/outside?

Just put a jingle bell on their collar. Seems too cruel to force them indoors.

difference of live length can reach ten years, this is half life of cat.