Mauricio, I believe like you "TDD doesn't magically guide you to good design", because it's not magic, it's focus.
Do you agree that TDD in itself can be seen as a frame that ensure you to think about your design often and check its resilience step by step ?
When you?e practicing TDD, for each small step, during refactoring phase, you've to focus on your design and how to improve it, this is how it leads to become better at design imo.
In my case I've observed, that the more I became experienced, the less I was breaking tests when modifying my code and the less changes were expensives. It sounds like the high flexibility you're talking about.
?
Gregory Salvan