[Still Thinking]

We had an old instrument that ran on DOS 2.0 and was still in use until the hard drive controller failed. No replacements available so a $100K instrument was turned into a boat anchor.

What's the big deal? Was the hardware some kind of special proprietary PC or something? You could easily build a DOS PC today. I occasionally boot my laptop in DOS to run some old software, and it works just fine. Also, DOS 2.0 should work from a current machine running a Virtual PC session. In fact I'm sure you could probably upgrade to DOS 5 or 6 and the app would still work fine.

[Still Thinking]

Or even a DOS window in Windows 3.1 or 9x (still had direct hardware access used by DOS programs).