> It sounds like Karen did a valuable service to the company. She combined a technical skill set with her domain knowledge to create a system that was so successful that the company now depends on it. Cleaning up the technical debt seems like a task that's well worth the cost.

The technical debt here was solved by creating a complex Excel worksheet. The sheet is the solution.

For small companies where these Excel monsters get created, it is the #1 best way (read - cheapest) to solve technical debt, which, before Excel, was probably a bunch of arcane manual processes that took 5x as long with worse accuracy.