It is true that cats thrive when engaged. But it's very possible to engage indoor cats.
I've had indoor and outdoor cats. And I've had indoor cats that we started letting outside. The change in their behavior when they have access to the outdoors is palpable. They come alive in a very obvious and dramatic way.
This isn't really surprising when you stop and think about it. We seldom make the argument that zoo animals are living their best lives compared to their wild cousins despite the objectively safer conditions of a zoo.
Now, you have to still mind your indoor cat. They need stimulation (look up "eat hunt bathe sleep") and attention and a good diet and cat-friendly environment. But give them that, and there's really no argument that being outdoors is obviously better. It's more that the outdoors gives some of that stuff by default, so compared to a cat owner who doesn't fully invest in their cat, maybe there's a case.
But if you understand cats, you get that that "coming alive" isn't some "free at last" sort of thing.
[1] Outdoor cats die way earlier than indoor cats
Of course going outside is dangerous for cats. It's dangerous for people too but we still do it.
