Given the state of play with security, a connection to the internet is pretty much going to always be more vulnerable.
This is about nation state adversaries. There should be no illusions that they laugh at people who think they are safe behind air gaps and routinely cross them. Often both infiltrating and exfiltrating data.
Would you prefer systems being very easy to attack or very hard?
You can have easy to administer systems that are also secure. It's not mutually exclusive.
I prefer security focused organizations attempted to deploy their limited resources more effectively. Well trained and security conscious end users are very hard to attack.
I don't understand your point. Everything is vulnerable; you can never secure something fully. Security in layers is how security works, and disconnecting critical systems from the internet seems like a good layer to add.