Earlier quoted context omitted.
Joel is making two separate claims there, though he doesn't cleanly distinguish them. One is that rewriting from scratch is going to give you a worse result than incremental change from a technical point of view (the « absolutely no reason to believe that you are going to do a better job than you did the first time » bit). The second is that independent of the technical merits, rewriting from scratch will be a bad co…
In Joel's defense, we don't know that gecko is better than X years of incremental changes to Netscape 4. One of the big issues with software engineering advice is that it is really hard to find apples-to-apples comparisons for outcomes.
Yes, we do, as does anyone who was following browsers at the time. They tried to incrementally improve Netscape 4 and it was a disaster.