Countless companies have successfully rewritten their products. Properly done, people outside the company never even realize that it was done. Joel has a sort of reverse survivor bias; those who fail are noticed, but those who succeed are ignored.

Worked for Mozilla.

I am utterly baffled by the fact that people are still posting that link as if it proved anything other than the fact that Joel Spolsky has absolutely no idea what he's talking about.

Or has this now become a running joke? Was the posting of that link ironic?

Was this comment ironic? Can you back up your claim, "Joel Spolsky has absolutely no idea what he's talking about," with some evidence?

Rewriting from scratch can be beneficial, take the V8 engine in Chrome for example.

Let's. Because this is confusing. What Javascript engine was the V8 rewrite of or did it experience a rewrite of itself at some point in time?

I'm not saying he proves anything but he certainly knows something about what he's talking about and the OP asked "why not rewrite" and Spolsky offers a number of reasons why. They might not apply here but they just as well might.

Count me in to the baffled crowd.