Shameless plug: I work for a startup ( https://www.sensorfu.com/ ) that helps utilities and industrial companies to solve that exact problem of keeping their control | ICS networks closed and isolated from internet.
Even larger utilities often have a rather small IT teams that are tasked with everything from keeping things running to change management, network design and architecture. And whether it is lack of time to focus on making the right things, or lack of time to even learn the latest right things<tm>, mistakes happen, all the time. And even when 'perfectly' implemented, it is far too often that we see that some one just decided that it's ok to run a cable from control room to open internet to make those night shifts a little less boring.
Even larger utilities often have a rather small IT teams that are tasked with everything from keeping things running to change management, network design and architecture. And whether it is lack of time to focus on making the right things, or lack of time to even learn the latest right things<tm>, mistakes happen, all the time. And even when 'perfectly' implemented, it is far too often that we see that some one just decided that it's ok to run a cable from control room to open internet to make those night shifts a little less boring.