As a cat owner who only lets their cat outside in a harness, I have some strong opinions about this, but I'll try to keep an open mind.
Some reading about the threat that domesticated cats cause when let outside with no supervision to other species
American Bird Conservacy
On Wikipedia
BBC
IFL!Science
Nature.com
Smithsonian.com
National Geographic
To quickly summarize all of that, the important distinction with feral and domesticated cats is, that a domesticated cat that is fed regularly at home will hunt for fun, rather than a feral cat that will hunt to feed itself. This mean a domesticated cat will hunt multiple times the amount a feral cat would, and domesticated cats are the biggest threat to the extinction of many bird, reptile and rodent species.
Cats may also hunt someone else's pet, even if that pet was kept in the owner's yard (My both rabbits had to be very keenly supervised due to neighbourhood cats coming in and trying to kill them)
Then of course there is the threat to the actual cat itself, that it will be run over or tortured by a cruel individual. In rural areas they can also be nabbed by lynxes (very common in fenno-scandinavia), Wolves and bigger birds of prey such as owls or eagles.
So...Do you let your cats wander freely outside? Are you aware of the enviromental impact this has on the world?
Also a kind of side question, does your country have laws about times cats should be kept inside? My country (finland) has, but those are not followed through. Basically you should keep your cat in check from spring-summer until the nesting period of many birds is over. However practically no cat owner I've ever debated/talked to knows or cares about this law.