This. Suppose you want to rewrite. Your task is to create a new, cleanly written program in a new language that produces the same output as the old, slightly messy COBOL. It must work identically for all appropriate inputs. There is no documemtation of the code and insufficient documentation of what combinations of inputs must be supported. No one understands the business logic fully. And there are 500,000 lines of copy-paste-modified code to convert. Any screw-up will likely cost more than your annual salary. How do you sell the risk-vs-reward of the rewrite project?

COBOL will outlive us all.