But didn't Apple actually attempt a full rewrite of the Mac OS which eventually failed? So instead they bought an already working OS and adapted it for their purpose.
That is not what Joel means by rewriting from scratch. NextStep was not written to replace Mac OS, it was just later adapted into that purpose. It is a completely different scenario.
Oh yeah. I remember that clearly. It was Copland- er, MacOS 8.
What a cluster----. Back then, crashes would result in a special kind of debug screen called MacsBug[0].
When you walked into their "release-ready, hands-on lab," almost every screen was displaying MacsBug.
The change didn't actually happen until NextStep became Cocoa.
I was also at a Microsoft "Longhorn" prerelease event. They were showing "live code demos," but you could clearly see the presenter quitting Director, when they were done with their demo.
That became Vista, another famous success story (but at least, it did ship).
"The Copland development effort is associated with empire-building, feature creep, and project death march. In 2008, PC World named Copland on a list of the biggest project failures in IT history."