I wouldn’t say that this has been true in my experience. I’ve worked on multiple products that evolve over time and it often happens that a new feature touches on previously built features that can’t support all the behaviors of the new one, so then a refactor of the old code (at least some of it) is necessary. You can argue that doing so would require regression tests, but that’s why you automate those with a unit testing framework. This mindset of never modifying previously written code within reasonable and pragmatic boundaries is so rigid and anti-growth, I don’t even know how it came to be so popular within an industry that identifies so closely with innovation. That’s how you end up with putting band-aid solutions on top of each other and slowing down the agility of your teams over time.