There ARE counter-examples of successful systems re-built from scratch.
The rewrite might have killed Netscape, for example, but without the rewrite from scratch not only would have Netscape died but there wouldn't be a Firefox or Mozilla Foundation now.
(And I'd argue that it wasn't the rewrite that killed Netscape: the rewrite was necessary. What killed Netscape, and would have killed it even without the rewrite, was that the then all-mighty Microsoft decided to get into the internet game for real and build a good-enough browser. It's ironic that this good enough browser was IE6, which we know view as the worst impediment to web progress).
Also, didn't Frederic Brooks say "prepare to throw one away"?
Why do people keep saying such things regarding previews and demos?
He is not complaining about a bug. Nor does he complain about a not ready for primetime feature in some early development version. Those are expected.
He is complaining about how a feature is implemented. Features such as this, rarely, if ever, change between preview/demo and final release. If it does change, it will be mostly because of the outcry, not because it was supposed to change in the final all along.
For a complex system, like an OS or a browser.
This is a bloody social news site, as trivial an application as it gets.
One man can write a half-decent one in a week.
And for ramping it up, even the scaling roadblocks and tricks are old news by now to the web building community in general.