I'm literally suggesting that some company could make a little device designed to be inserted inline between an IoT or industrial device and the LAN/Internet that would, effectively, retrofit a security policy onto the device without interfering with its normal operation or requiring rework of the device. You're saying that plant operators could use an off-the-shelf firewall, too. I'm saying that using an off-the-self firewall is both weak and a pain to configure and that I think a bespoke device could do a much better job for quite a few reasons.

I don't think any goalposts have moved.