The world changes little-by-little, not all at once.
Let's say your application (100k LoC of Cobol) handles payroll. Your government introduces some new rules about how paternity leave should be handled. You have two options, rewrite the 100K lines into Java and add the new rule, or just add the new rule in the Cobol app. The latter is cheaper, at least in the long term, so that's what you do. Now you have 101k lines of Cobol.
20 years, four tax reforms, three mergers, five international expansions and fifty lawsuits later, you have migrated 100k lines of Cobol to Java, but your app grew and is now 3 million lines.
Let's say your application (100k LoC of Cobol) handles payroll. Your government introduces some new rules about how paternity leave should be handled. You have two options, rewrite the 100K lines into Java and add the new rule, or just add the new rule in the Cobol app. The latter is cheaper, at least in the long term, so that's what you do. Now you have 101k lines of Cobol.
20 years, four tax reforms, three mergers, five international expansions and fifty lawsuits later, you have migrated 100k lines of Cobol to Java, but your app grew and is now 3 million lines.