It applies to any old project.

Imagine that you need to replace a live system, downtimes are not allowed, probably provide a new UI while offering 100% of existing features.

There is no documentation available, or even if there is one, you won't find anyone in the building that will put their hand on the fire for how much of it actually maps to the running application.

So a project of reverse engineering existing application into a requirements document or updating the existing documentation usually takes several months.

Now just for a very basic project cost, consider the monthly salary of everyone involved, multipled by the amount of months.

And we are just speaking of phase I, then there is the actual re-write, followed by verification that 100% of the old features are still present on the new system.

Naturally it is cheaper to keep existing systems running.

This applies to any old platform, not only COBOL systems.