I know of a small company run by a retired man, that designed and installed several industrial control systems. All of these are air-gapped and run by very old

I know of a small company run by a retired man, that designed and installed several industrial control systems. All of these are air-gapped and run by very old computers on DOS 3, I think they might be 16-bit. It's run by him and him alone. He travels out to fix them in different cities once in awhile. I asked in passing if he tried moving to VMs, he has but didn't describe it, he said it didn't work out.