Agreed about the context being key. It always is. My point was more about adding incremental value to your customers with solutions to small problems rather than going after all their problems with one big release.
It's hard to say whether Copland was a rewrite or a refactoring, but what is indisputable is that the root architectural decisions of Mac OS were a dead-end.
They actually had to go beyond a rewrite to build something completely new and different, and then retrofit an emulator onto it to support the old system.
"Microsoft almost made the same mistake, trying to rewrite Word for Windows from scratch in a doomed project called Pyramid which was shut down, thrown away, and swept under the rug. Lucky for Microsoft, they had never stopped working on the old code base, so they had something to ship, making it merely a financial disaster, not a strategic one."
Even that wasn't "starting fresh". The starting point was a complete operating system: OPENSTEP. It was just a different operating system than the one they were using before.
They wrote a new system in tandem with maintaining the old one and when it was mature enough, swapped out old for new. This pattern is very commonplace.
They were two different products that ended up converging enough that sharing code made sense.
On the subject of rewrites, with the benefit of hindsight it's easy to point out when it was a bad idea. But clearly it doesn't always have to be that way. After all, it never hurt Microsoft on occasion, and Apple.
As almost always, context is key.