In most cases, it's running millions of lines of legacy code that interacts with all sorts of crufty, decades-old proprietary software components. (iow, it's not COBOL itself that blocks a migration, it's the fact that real-world COBOL code tends to rely on this terrible proprietary stuff. I mean, it's nothing that couldn't be dealt with cleanly if a gigadollar was literally on the line, but I suspect that such figures are somewhat exaggerated.)