These DOS machines for industrial control could probably be replaced by an Arduino or a far more reliable MCU, whereas running an actual legacy PC as a business-critical component in manufacturing has to be a bit of a nightmare by now. AI could probably do a good enough job of working out how the legacy DOS executables were intended to work.

This isn't hackaday or adafruit. Everything is easy when you don't have to actually do it. You are wrong on every point.

You might notice that I never once claimed that the replacement I described would be "easy" or, for that matter, even advisable given the broader real-world constraints involved; just technically feasible in the barest sense. I don't think many people would want to use DOS to design a greenfield system of that kind today, and there's a reason for that. Yes, you can buy newly made "DOS PCs" today, but can you really ensure that today's brand new DOS PC will behave in every way that matters like the actual 30 years old DOS PC that used to control the machinery? That's not a trivial question to answer.

If you design the system from the outset to work with an actual PLC/SCADA or similar (the typical solution for hooking up to big industrial machinery of that sort) that's a bit less likely to come up as an issue, and the hardware will actually be designed for that kind of environment.

Yes, if you ignore everything that was discussed, invent time travel do you can "design the system from the outset" as the prescient you are, and pretend anyone was talking about greenfield, you get to be right. Good for you...some people just need the 'win'.

Given the implications, I guess nobody is going to touch those setups to put an SDL-based program on them, though...

Yeah...this is "if you screw around with it enough, you void the warranty and we will no longer support it" for a potentially multimillion dollar machine.