> if you take that approach when testing all the thousands of components together, which is simply just too complex to exhaustively model[1], you'll never get anywhere.
This is exactly why ideas like test-driven development don't work well as a general approach.
Most realistic systems exhibit non-linear interactions where correctness is not compositional. Local correctness does not compose upward in any meaningful sense. Top-down design (working backward from the customer) allows for you to perform what is effectively one big global search. Bottom-up design (TDD) requires many local searches that all have to fit together perfectly at the very end. With units & composition, the consequences of component A's interactions with component B may not be considered until nearly the end of the project. If you are testing an integrated vertical, you will discover these interactions much earlier.
This is exactly why ideas like test-driven development don't work well as a general approach.
Most realistic systems exhibit non-linear interactions where correctness is not compositional. Local correctness does not compose upward in any meaningful sense. Top-down design (working backward from the customer) allows for you to perform what is effectively one big global search. Bottom-up design (TDD) requires many local searches that all have to fit together perfectly at the very end. With units & composition, the consequences of component A's interactions with component B may not be considered until nearly the end of the project. If you are testing an integrated vertical, you will discover these interactions much earlier.