An internet air-gap is probably enough for a vast majority of use cases.
There's lots of talk about engineering here along the lines of "good engineering is knowing how to make a bridge barely stand up", but in Security, especially IT sec there's often little discussion about real risk and impact. And striking a reasonable balance.
Places I've worked consider their product and information high security whilst embargoed (mostly financial). The IT security at these companies matched that posture. But people all drank together, shared everything over drinks and had terrible personal security.
I'm not a security skeptic at all, I just think that the simple stuff goes a long way and that it's somewhat unhelpful to compare regular IT use to CIA style IT use.