I think I'd have to offer that my experience differs. TDD is not at all big-design-up-front, even with this reductive exercise. In fact, most features start very minimally and the tree of dependencies grows over time, just like any system becomes incrementally more complex. TDD is just one tool (of many) to help manage that complexity. Both by offering some regression value (at least of the logical bits) and also by encouraging small, bite-sized units that are easy to make sense of (and therefore change or replace)