It's pretty easy, just ensure that all cats sold for anything apart from breeding are desexed, and have significant penalties for allowing unregested animals to breed.
PAWS in Seattle will give you a cat and it comes for a coupon for a free spay or neuter (often kittens are too young at adoption time). Of course the fee for adoption is higher than the cost of the surgery, so you are paying for it whether you use it or not.
There are often very cheap city/county options too: where I live its less than $30 for a spay/neuter + rabies vacc (significantly less than a private vet would charge).
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)
So it's a bit like saying "instead of ROHS can't we just mandate recycling, but if the cheap kids toys people dump indescribably could breed.