[hn_throwaway_99]

Google isn't the only example here. Heck, just look at the transition from Classic MacOS to OS X. I definitely think Apple would have been dead long ago if they said "A rewrite is too expensive/risky, let's just incrementally improve Classic MacOS".

I guess my point is that there are right ways and wrong ways to do ground-up rewrites, and the fact that a lot of people do them the wrong way shouldn't mean the lesson should be to never do them.

[Kye]

Replacing one mature operating system with another, which itself was based on one even more mature, and adding stuff is not quite a rewrite. That was more like how Microsoft moved NT into its consumer OS. It was a bit of a mess but had a clear payoff once everything was updated or obsoleted. NT and BSD were both battle-hardened long before anyone thought to put them in consumer systems.

[goto11]

Apple actually tried to rewrite the classic OS from scratch with the Copland project and it failed (at enormous cost). OS X was not a rewrite-from-scratch since they acquired an already working OS from Next. The had to add compatibility layers (which was in itself a major undertaking) but they didn't write the OS from scratch.