Colonial Pipeline precisely does keep it's control network disconnected from the internet - the only thing that was ransomwared is their corporate network. They shut the pipelines down voluntarily to prevent further spread.
Many industrial SCADA systems (nearly all) send data from their "OT" systems (PLC/DCS/SCADA) to their "IT" and business layers (Historians/Timeseries Databases, Dashboards, Power BI/etc). This almost always happens through a two-way link (think TCP/IP, HTTP). While the software should not allow data flow backwards, the hardware absolutely does. So how much do you trust the software?
I often advocate that industrial SCADA systems utilize "data-diodes", one-way opto-isolators, or other physically verifiable methods of confirming that no information/data/instructions can get from a "higher" layer (OSI Pi, PowerBI) to a lower layer (Allen-Bradley PLC, Siemens PLC, Emerson DeltaV DCS, etc).
Convincing the powers-that-be to do this has been incredibly impossible in most places and a large reason why I'm trying to transition to a different space - I simply have had ethical concerns about providing engineering services to critical infrastructure without building in best practices.
Stuxnet was over a decade ago - I don't understand how these protections aren't mandated by the DHS already.
Disclaimer: I don't think it's reasonable for non-involved people to assume the OT side has been compromised. I do think Colonial will need some time to verify the integrity of their SCADA systems and it makes sense to keep the power to the physical devices (valves/pumps) offline until they do. I understand why they chose to shut down but I don't think there's any evidence that they'll be unable to start back up again.
Lastly, I saw a quote in one article:
>>> Digital Shadows thinks the Colonial Pipeline cyber-attack has come about due to the coronavirus pandemic - the rise of engineers remotely accessing control systems for the pipeline from home.
I strongly doubt this. It's possible, of course. But it's extremely unlikely to me that employees would have remotely accessed OT/SCADA systems from home. No one I've worked with has had that capability enabled.
Many companies use products which have been shown to have flaws, like Citrix or various corporate VPNs. These could be compromised to get access "closer" to the OT layers but never directly into it.
Onion layer security is very much practiced everywhere I've been.
Edit: I have heard of some petrochemical facilities moving towards allowing operators and engineers to manipulate valves/pumps on their iPhones. This horrifies me for many reasons. I've never actually seen it implemented and I always bring up Stuxnet when I hear people mention it. I personally believe that DHS should make this sort of thing illegal for critical infrastructure. Many good engineers disagree with me.