Counterexample: SpaceX. Sometimes, starting from scratch can be very helpful.
This is still starting from scratch. The design may be conventional, but you're benefiting from a modern approach to processes and design from the beginning.
I mean this is the best way possible, the Falcon 1 didn’t reinvent the wheel. They basically said what’s the simplest useful rocket for getting stuff to orbit copying as much of the design as they could, and built that.
I'd say that if you are gluing together a bunch of known-good components into a specific configuration that isn't a full copy of something else, you're building from scratch.
Take say all those identical looking highway overpasses. Even if one of those projects started with a blank file, applying standard solutions to standard problems isn’t starting from scratch.