The issue with not rewriting or rebuilding things is that you miss out on the opportunity for conceptual changes and new approaches to solve problems in more robust and elegant ways. For example, Unreal Engine 6 will depreciate and eventually remove it's existing Actor based scenegraph with an entity component system. This is essentially a rewrite of the core of the engine, and it's a massive conceptual change away from the old style of inheritance-based OOP game programming to something which is much simpler to reason about and provides optimisations which are not possible with Actors. They aren't doing this out of delusion, it's that there are fundamental limitations to their current approach which are not solved by simply refactoring - it requires an entirely different conceptual approach which turns out to be simpler and better in pretty much every way.

The problem with never rewriting is that you miss the chance to discover better ways of doing things. No system is ever built perfectly, if all you ever do is iterate on a bad foundation you're limited by that. Approaching a problem in a different way is only possible when creating something new.

Of course you shouldn't rewrite something just to do it the same way, and you shouldn't rewrite something just because you don't understand it, but I also don't understand the perspective that you should never build something new when there is an existing solution, because it's often only through building out one or more bad solutions that you arrive at a good one. And if you follow such a strict rule, you never end up building anything good.

In Unreal's case, they built the Actor system and have used it for almost two decades. In that time we have discovered better ways of doing things, and it would be a shame to be stuck with an inferior design because Joel said to never rewrite things almost three decades ago...