[kohanz]

You're getting closer to a business case, but if you're confident in your assessment, you need to put it plainly, in terms that management can digest quickly. The bottom-line is money, and in engineering, time = money.

So here's the case: the time to add the new features on top of current implementation + the time to support this new, perhaps brittle, version = X. The time to refactor the application and add the new features + support the new, perhaps more robust, version = Y. You must show, in reasonable detail and with confidence, that X > Y.