What, specifically, does this application do?
Likely, requirements and needs have changed in the last 20 years.
As a developer, I've seen that sort of requirement dozens of times.
"We need to keep this old program running, because we can't train them to use something new."
Not realizing that the only reason the old program seems optimal is because they're so used to it. Even though it is actually a really crappy application.
Not understanding what is actually possible with something new. I've rewritten old applications that were supposedly critical and irreplaceable.
Result? More functionality, and about an hour of "training". Click here, click there, done.
What does this thing actually do?
About the only time where it is critical to keep some old thing operational is if it were running some old, obscure and proprietary piece of machinery.