I'm not familiar with the architecture of the app in question but are the UI parts and business logic/processing parts not properly separated such that a full app rewrite is needed? Someone referred to the backward compatibility of Windows apps as a curse: this is especially true when companies finally decide to update a VB6 or early 2000s WInForms app to something modern, the first step of which is to extricate all of the business logic in click handlers. By the time Cocoa came out the concept of separating your application into logical layers such that things like the UI could be swapped out with newer tech faster were well known so this shouldn't have been such a big deal if the app was constructed correctly.