[ignorabilis]

As long as there is some sort of logic tests can be written. The question is can we prevent writing tests at all by designing better our applications and by taking advantage of FP languages and tools like the REPL? And I believe at least in theory we can. In practice life happens so this theory is certainly not applicable to every case on the planet.

When things are decoupled refactoring is not that dangerous. If all pieces of your program are small, intelligible functions you can always just refactor a single, simple function and you should not need tests to rely on - all you have to do is make sure that this function is working like it worked before the refactoring (or better).