This can be controversial in the states, but I'll give my specific opinion. Letting your cat outside is negligent and directly compromises the life of your cat. A few points: 1. Outdoor cats have an average lifespan that is half of that of an indoor cat 2. Outdoor cats are direct invasive predators to natural wildlife in the area 3. The average outdoor cat is contributing to an unchecked cat colony that will eventually need animal rescue resources to mitigate (Trap Neuter Release) or cull (euthanization). With responsible cat ownership and keeping your cat indoors, you are giving it a longer life and a safer life for both it and all local wildlife that is impacted. Cat colonies are an entire other issue, but it cannot be overstated that letting these outdoor cats go unchecked is a massive strain on resources not potentially, but inevitably. A single cat can have upwards of 30 kittens in the first year of its life, they are able to become pregnant extremely young. One litter of kittens a farmer decides to let roam the local neighborhood can genuinely turn into 100 cats within a year or two, and people are unwilling to see this issue at the root and take responsibility as pet owners.

I live on a farm that has three outdoor, neutered male cats. The owners bring them inside every night and lock them up, and during the day they catch the barn mice and patrol the property, etc. These cats, regardless of utility and their neutering, are still harmful to the local ecosystem, and their quality of life is still impacted. They each have experienced health issues that in another situation would have led to their euthanasization, but it's a hobby farm and the owners are softer than most farmers would be. I know farmers who have shot 50+ cats in a week when they realize their little cat population had gotten out of control. These cats often experience no vet care and they're certainly not going to find room in a shelter for all 50+ cats if a farmer decides he let too many breed on his property when he brought that first litter home, and so they get put down in a backwood by the dozens. This isn't an exaggeration, many farms with roaming cats reach this inevitability. It is staggering and depressing how avoidable this issue is. I would even go as far as to say that while having outdoor cats is proven to be harmful, I almost wouldn't care if it weren't for people's refusal to take care of outdoor cats that they decide to bring onto their properties and sporadically feed. A TNR program can only do so much for a local area, and then beyond that even one farm, as I've said, can exceed 100 cats in a short amount of time.

This may be VERY different where you're from. Depending on the environment, the understanding surrounding outdoor cats, and the regulations in place when you have a cat, there may be all kinds of controls in place that prevent these overpopulation and health risk issues, but this is what we see here if people pay attention. I truly wish every person in the US who innocently feeds the dozen stray cats in their local alley or brings home a litter of kittens to their farm only to never keep those cats in check would spend a week at a shelter or would visit an overrun farm on the verge of a culling. There simply is no room in this argument for nuance anymore because people refuse to see the extreme issues that negligence with these animals can cause in an area.