This principle works most but not all the time. I develop an interactive climate-system model that evolved with many small steps over 23 years. So it has many patchy fixes as climate policy structure changed, gases and sectors added etc. Then converted from java to scala module by module (out of ±50) , each step checking the plots looked as before. Result is it works, but parts are messy with legacy options and outdated code style. So sometimes it’s necessary to radically rethink the structure, take big bold steps before it works again, that’s hard. Scala strong type system, with hints from compiler (and “metals”) help make such refactoring easier.