> More importantly, no changes over the last few years means that absolutely no one in the company has in-depth and current knowledge of how that code works, so a refactor is pointless because no one knows what the specific problems actually are. reply

That’s is the actual reason you might need to rewrite it. If your critical system is written in an ancient script that nobody can understand, and does no longer supported, and as a security risk, at some point it will simply stop working. And there will be nobody that can fix it.

Yes, rewrite may be painful, but if you can no longer find the people to support the old thing it may be necessary