There seem to be large cultural differences and they have their reasons.

For one, the average area of a house in the USA seems to be above 2000 square feet, whereas here in the UK it's more like 750 or so.

It feels worse for the animal to be cramped in a tiny space all their life, so we let them outside. Also there are no predators apart from the occasional domesticated dog, so it's safer. The biggest risk is traffic, which is dependent on where you live.

The bird thing brought up every time is weird to me, because no one seems to care about birds until cats are part of the conversation, suddenly birds are the most important part of the ecosystem.

We learn new things over time. Birds are wildly important to the world's ecosystems. House cats, not so much. They're more useful to humans in keeping rodents away from our food but that's not as necessary these days.

The supposed impact on bird life seems possibly quite overstated. Extensive personal experience suggests that (at least the cats that select me to live with) do not catch many birds at all.

Of the current cat with free-ish outdoor access: Last year birds accounted for 3.6 percent of all recorded catches (5/138). This year birds are 1.7 percent of all recorded catches (1/58). And all of the birds I've seen caught first knocked themselves out striking windows, or flew inside the house via open door and injured themselves trying to escape in panic. The healthy ones don't let cats near them at all.

Of the prior cat, zero birds over 9 years. Of the cat before that, maybe 1 or 2 birds over 18 years.

Of course anecdotes aren't data etc, but, the impact of data is its story, and anecdotes direct and constrain the collection and analysis of data.

how are you going to be like "my anecdote is actually important" with an N of magnitude 100. long-term and wide-scale research and monitoring has tracked millions of birds and have used models to extrapolate to the billions. your comment means little more than nothing. have you considered you haven't seen the extent of what the roaming cat has killed? it won't always bring it back to you. and why is it that the non-bird animals aren't important? biodiversity reduction, not just birds. and you think that even if they are sick or injured catches that your cat isn't impacting the other animals? you realize other animals gotta eat? if they can't b/c your cat takes it, or worst infects it with a parasite by chomping on it, your local predators are gonna suffer.

I grew up on a farm (which my brother now owns). So my anecdata spans from roughly 1980 until today. The farm cats tended to have two very different life spans. The short lived ones died within their first 3 years. The long lived ones lived 16 years or longer. With most bunched up around 18-20 years and the oldest two living to 25 and 27 years.

We've had no cats that lived between 3 and 16 years.

I have no idea why. To some degree you could get a sense which category a cat was in within the first year or so.

If I were to guess I think a cat will probably have a shorter life span in a city. More cars and more opportunities to get infected with diseases from other cats.

I live in a rural area with indoor/outdoor cats and I've seen that too, honestly it feels like a pretty classic bathtub curve, or that curve showing experience vs survival whose name I'm spacing on, the one with the dangerous "knowing just enough to be overconfident and kill themselves" valley. Some cats are inherently more cautious, but in general over the decades my observation has been that in general cats are like humans who engage in risky but rewarding behavior be it work (factory, or devops for that matter cough) or sports such as rock climbing or kayaking. When someone (or some cat) is absolutely brand new to the world, nearly everyone is very careful. Then they grow in comfort and experience, having learned a lot of the basic ropes, and in turn get more and more confident. But they haven't had their first crisis yet. Eventually comes the inflection point, where overconfidence leads to some sort of life/death (or losing the production database and holy shit are you fucked) moment with a rare event.

Those cats (or people) who make it through that crisis will gain some hard humility forever more and seem to go on for a long time after. Those who don't, well, obviously that's it. I had two big male cats back in the 90s, who being tough young males naturally once they got to 2-3 years old felt pretty confident. First of them sadly vanished and never saw him again. The second a few years later got into some fight or flight with/from something, I don't know what, but there were shrieks. But he escaped/fought and I found him. Forever after he was very careful, he'd go out the door onto the porch and whereas before he'd just instantly head off, after he'd immediately sit down and observe/listen for a good long while first. He lived to 21 years old. Another outdoor/indoor lived to 19.

It is truly hard to take the risk, but they're so much happier for it. We all reach the same end in the end, so I feel very strongly that there are limits to how far one should take "being safe". And there are good ways to massively reduce the bird issue. I love birds as well, but 3-5 caught per year when I'm feeding and support orders of magnitude more birds than that seems acceptable.