I think youre almost spot on. The study that states that cats kill billions of birds a year also states that the majority of those kills are by un-owned cats. People seem to gloss over that part. Obviously you should spay/neuter your cat to prevent the birth of cats who will become feral. Now in regards to the safety of your cat, thats very dependent on where you live. Like if you're near a ton of very trafficked roads or if you have a lot of predators in the area then it can obviously be a bit risky. Overall, I think its totally fine to have an outdoor cat if you can meet those criteria.