[IAmEveryone]

The "parts of it" in your question might be the clue to an answer: you start implementing some often-used code path only to realize that they share code with all the rarely used code paths (permissions/authentication come to mind),

So you'd need to reimplement a whole bunch of your codebase, at which point you'll have two versions of everything that you need to keep in sync.

[ta988]

That's exactly why most COBOL rewriting projects in banking have failed miserably. It is much more complex than just rewriting a complex CRUD app. It is not just about the main program, it is also everything that connects to it and expect it to behave in super specific ways.