This is no different - either way you are buying a system that includes controls. While separating this from the internet sounds great, in practice internet control is too useful to run without. Maybe you put in a few firewall rules to protect things, but these often are lose enough that a hacker and bypass them (by looking like a legitimate access - since the people who need to access this will want to work from their cell phone)

There’s always the issue that it might be hard to find competent people that can implement a control network isolated from the internet if it has an internet connection by default.