My current cat has almost 3 acres to roam around in - if he so chooses. He rarely does go out - perhaps once or twice a day for about 1/2 hour each time - his choice.
We may be moving (fingers crossed!) to a more rural area and on a cul-de-sac. If we do, the garden will have cat-proof fencing put up. This is because there is a very popular dog-walking area adjacent to the new house and, with it being common land, people rarely have the dogs on leads. So, to be on the safe side, (I have lost a cat to dog attack and the cat was on my property at the time!) I'd rather keep him contained to the garden - although said garden is over an acre in size so I don't think he'll be complaining too much!
I know a lot of people with cats who have either cat-proofed their fencing or built 'catios' for their little 'darlings'. Both, to my mind, give the cat the opportunity to express natural behaviours whilst keeping them safe from theft or accident.
Not sure if I agree with a cat being an indoor cat if it is healthy. I realise that for some (eg FIV positive) it is the only option other than euthanasia but for the rest...