Aren't you two saying the same thing? TDD works when the requirements are well defined upfront, which in the case of fixing existing system is provided by the system itself. But writing new programs is exploratory in nature: you don't know the exact requirements or shape your software will take, and TDD shifts a lot of precious money, time and attention from the what to the how. Which to me, I agree with you, is an engineering issue before being an economic one.