If this were tried, the present copyright holders would come out of the woodwork and enforcement would happen pretty quickly.
I'd be pretty surprised if a judge was like "ah yes the defendant's consultancy modified a piece of industrial control software that you haven't given a thought to in three decades to make it run on a modern computer and not require a parallel port dongle, and that's definitely a DMCA violation and you've been harmed by it and deserve all the money."