No, with MVC you don't need to rebuild the entire system. MVC separates concerns so the API can be added without changing core logic. The Model and Controller layers are decoupled from any presentation layer (web UI vs API). We can: 1. Add a new API Controller that calls the existing Model classes. 2. Return data from the Model in a format like JSON instead of HTML. 3. Expose the same business logic to third parties without changing how the app works internally. The View layer is the only thing that needs changing to support the new API presentation. MVC makes it easy to add new interfaces without rewriting core code.