There's nothing cruel about putting a leash on a cat, provided it's done right (with a proper harness), and the cat accepts it.
Some cats, as you've seen above, love it because of what it means to them. For those, it's a wonderful idea.
Some are just not interested in "this stupid human contraption, thank you very much". For those, there'd have to be a very good reason for insisting, else it would be cruel.
My cats are indoor/outdoor, so getting a breath of fresh air is not an issue, but I am gradually making some effort to suggest the idea to the one who is most likely to take to it (Fawn), so that she would be able to visit my Dad with me, in his extended care facility. Animals are welcome there, but of course one needs to be able to control them, and given that they have resident cats, I wouldn't think of taking one of mine in there unleashed.
First the harness. Fawn nearly freaked the first time I tried it, but I left it a while after that to let her "get the nasty taste out of her mouth", so to speak, and she was better the second time. I don't know whether she will eventually accept it or not, but if she does get used to the harness, then we'll work on adding the leash, and maybe eventually she'll get to go see Grampa. However, it's not something I'll be insisting on, since it's not something that NEEDS to happen.