There’s nothing natural about a domesticated species being spread around the world and allowed to roam free while simultaneously receiving care and protection from humans.

Cats can live happy lives inside, they just need a bit of attention.

Ours get a couple of hours supervised outside time whilst on long leads. They can and do carry out hunting behaviours, but being on a tether and being supervised allows us to dramatically reduce their impact on wildlife to almost nothing.

Considering what they do attempt to catch whilst we are standing their, I'd imagine they would take a dramatic toll if we just let them roam free.

And only implied in your comment: they should not be alone. Not an excuse to go full catlady as 2 are plenty.

They are (in nearly every country) a foreign invasive species, so “naturally” they shouldn’t be there at all.

Aren't all animals?

I guess the ones meant to live in caves or underground would do fine indoors?

Not in North America!