Surely every plant like this would pay something for the ability to run on modern OSes, and presumably there are enough plants using this kind of software to recoup the cost of development?
Moreover, for software that was put in place 20 years ago, it's likely that some of the functionality is unknown, i.e. the whole system works together, but exactly what is being done by each piece of software isn't fully understood by anyone still at the company. So it would require a bunch of work to even spec a replacement.
So it's totally rational to keep the existing, working system, even if you need a dedicated PC to administer it.
(Source: I was in a similar situation ~20 years ago.)
Context: I used to work on solar power management and control, and developed manufacturing processes to build our hardware.
For the first part, if the original company has gone out of business, and enough records and knowledge lost and scattered, then it'd actually potentially be very difficult for a new group to identify what the original customers were, much less identify what the potential size of market is today. That's quite a lot of risk to take on (in my mind anyways). Industrial automation is one of those things that people don't openly disclose in a way that makes it easy to canvas the market cheaply.
I've thought of reverse engineering the software but to tell the truth the program and the machine it controls were just built very well and so long as we have spare hardware it could go forever.
Due to driver problems I'm not sure you could even build a system these days that would be guaranteed future proofed for 30+ years.