the cobol system was already tested for years or decades, important bugs are already fixed, it's other quirks have became part of the workflow for people using it, and for other systems interacting with it, some of them likely also cobol. if you write a new system, it has to work the exact same way. same features, same bugs, same everything. but realistically it will have brand new bugs, it will miss some functions and do some others differently enough that it sometimes causes problems, and nobody knows why.