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.
