Hard disagree. The system I'm talking about is the largest scale system I've worked with (millions of QPS, thousands of servers). We built it in such a way that we were able to rewrite entire parts of our stack easily and relatively safely. The techniques and tools to do it are widely available today.

Good for you. Modular programming goes a long way, of course, but I'd say that your changes were then those that could be foreseen, for example by drawing the right modular boundaries from the start.