the GP is saying that across a sufficiently large population... you will have cats escaping (some subset of the population), and thus a feral population will ev

the GP is saying that across a sufficiently large population... you will have cats escaping (some subset of the population), and thus a feral population will eventually form. It does not necessarily need to be your cat that kicks this off. One trustworthy indoor cat steward can get away with it; a large enough population will naturally contain some careless people, or even surprises (hit by a bus)