It depends at which level you're at. If you're working at an infrastructure or firmware level, throwaway work can be expensive and time consuming. Re-architecture can also be time consuming. There are some parts you have to spend design cycles on, especially if you're doing a large project. It's not that you can't pivot, but the pivot takes a lot of time. At the very least have a set of System Features and System Guarantees that you will drive towards.