First assess the business value of the system, then choose the technical direction.
Building from scratch may sound cleaner, but it often means losing proven processes, extending implementation time, and increasing business risk. Modernisation can be the safer path when the system already supports active users and important processes.
A good audit should separate what needs immediate repair, what should be rebuilt step by step, and what can remain untouched. That way decisions are based on priorities and costs rather than frustration.
In practice the best strategy is rarely extreme. Stabilise first, clean up the critical areas, and then introduce new modules on a healthier architecture.