There is no difference between a domestic cat and a wildcat, as in they're able to interbreed because they're genetically the same species. So if you're in a region where they would naturally exist (effectively all of Eurasia) then it's perfectly fine to let your cats go outside.
TLDR: It's only bad to let your cats out if you live in the Americas or Australia.
It's bad because of the sheer number of them. A single cat can kill over a million animals over its lifetime I believe. A single pair of cats can produce around a thousand cats over the course of a few years, so that adds up. I love cats but they're absolutely an ecological disaster if left unchecked.