You should have a TDD friendly design before actually use it. But most of the project starts with "let's make a demo to try this idea". And after several years

You should have a TDD friendly design before actually use it. But most of the project starts with "let's make a demo to try this idea". And after several years, with thousands of patches applied, the demo finally becomes a product. When new devs join in the team, "the projects even don't have any unit tests, why do we need TDD?"