For example, let's look at the recent major Colonial Pipeline case. Their pipeline systems weren't connected to the Internet, and did not get compromised. What got compromised was their business billing and customer communications systems - and those do need to be connected to internet, that's their whole point, and they apparently were critical enough to make them shut down the (uncompromised) pipeline anyway.
It doesn't matter if your meat packing plant machinery SCADA systems are isolated, your inventory, logistics and sales systems are critical for your operations and need to be connected to the internet, so a ransomware attack will kill you even if your plant equipment works fine.
It doesn't matter if your chemical plant sensor network is isolated, your payroll and shift scheduling system is critical to your operations and needs to be connected to the internet.
Heck, for so many companies their email systems are critical to their operations (and leaking the contents would cause a massive liability) and those obviously need to be connected to the internet.
Not connecting is helpful in some cases, but it's nowhere close to a sufficient solution.
