You’re looking at salaries for COBOL programmers in the UK which is a small subset of an already small market.
And I'm sure some consultants can earn vast sums fixing old COBOL code, but that's just because they're consultants. Consultants always earn vast sums.
I don't understand why people are claiming this sets your career to only ever write COBOL. I know plenty of people who started their careers with perl, but have been Java, JavaScript, TypeScript, Go, or Rust programmers at different points as their careers progressed.
Someone overseeing a project migrating a COBOL monolith over time to a set of decoupled services in some other language would easily have a strong story for the timeless need of improving application architecture.