A lot of water infrastructure is physically spread out. It be very expensive and cumbersome (and probably inefficient) to require staff to by physically present at each site for monitoring and making any changes.

Keeping things up to date and holding critical infrastructure to higher standards than consumer tech is not a bad idea.

Taking things offline and properly airgapped can also work, but wouldn't the cost of that exceed making specialized things and maintaining them?

We got into this situation due to cost, not ignorance. Both choices are higher cost than putting ancient devices on the internet.