Except COBOL systems are often easier to formally verify than modern systems.

They're often simpler, and whole classes of security vulnerabilities are mitigated by design. My favorite example is SQL injection mitigated by having proper access controls in the DB. Why prove about every SQL statement that it cannot be nefarious, when we can just prove it about the connecting user? The user access carries the intent of what the application program wants to do with the database, in a separate channel, independently of the complexity of the application.