Ah, I see Americans are bitching about people in the UK letting their cats go outside.

Firstly, even the biggest UK bird charity agrees they have little to no impact on bird populations. https://www.rspb.org.uk/birds-and-wildlife/advice/gardening-for-wildlife/animal-deterrents/cats-and-garden-birds/are-cats-causing-bird-declines/

Secondly domestic cats have been in Europe for far longer than they have in the Americas, Australia and NZ etc. They fill the niche of European wildcats which have already been driven away from areas where humans live by human activity.

There is a risk of domestic cats and European wildcats from mating with each other to create hybrids, however this is rare as domestic cats tend to be neutered/wildcats don't live near humans and therefore don't live near domestic cats.

Thirdly, the UK is a rabies free island (except bats) so they aren't at risk of rabies.

Fourthly, there's no large predators here. Foxes can kill cats but it isn't common. Cats are also known to kill foxes here but generally they ignore each other.

The main risk to cats is cars. Rescues won't let people who live near busy roads adopt cats who could go out if they were in a safer area, generally they advise people near busy roads to adopt cats who have to stay inside due to health issues.

If I lived in the US, Canada, Australia or New Zealand then yeah I wouldn't let my cats out. But the UK and Europe are different, almost all vets and cat rescues recommend letting your cats go out (if it's a safe area).