Well, don't get me wrong, I've seen my share of codebases with "Iface" and "Impl" classes (ew), and stupid, counterproductive unit tests that were done to check

Well, don't get me wrong, I've seen my share of codebases with "Iface" and "Impl" classes (ew), and stupid, counterproductive unit tests that were done to check off boxes on a PM's checklist rather than improve stability. My favorite was a test to ensure that some static method somewhere returned the same hard coded string every time. The method was one line long, 'return "foo"; '. But. If you've defined nice clean interfaces, and a particular module has complex behavior that will have to evolve over time, then adding tests to certify that that behavior is correct shouldn't be too much of a chore -- and they save your bacon when you need to change it and track any regressions afterwards.