The massive real market here is enterprises that need to rewrite legacy code to modern platforms, retaining the business logic as-is but modernising the style.

.NET Framework 4.x to .NET 10, Python 2 to 3, Java 8 to <current version>, etc...

The advantage the LLMs have here is that staying within the same programming language and its paradigm is dramatically simpler than converting a "procedural" language like C to an object-oriented language like Java that has a wildly different standard library.