I once heard someone describe legacy systems as systems without test coverage. I think it’s not the best description, but it’s certainly an interesting perspective.

It’s part of why it bothers me when folks say they use LLMs to make unit tests. If anything, you should be writing tests by hands to get a solid specification then let the AI make the code. Of course this is a false dichotomy, but I’m just saying if you have to choose between those two options in some weird hypothetical bizarro world.