"That's a meme started by a Joel Spolsky article, and maybe an allusion to the "second system effect" (which is about something else altogether)."

That said, the "second system effect" is not about merely rewriting risks, but especially about architecture and design choices. From the very wikipedia article:

"""People who have designed something only once before, try to do all the things they "did not get to do last time," loading the project up with all the things they put off while making version one, even if most of them should be put off in version two as well."""

That is, if you design your rewrite _without_ wanting to build a bigger, more involved product, but merely a cleaner and more cleanly made product, this does not apply.

Another quote from the very article: """The second-system effect refers to the tendency of small, elegant, and successful systems to have elephantine, feature-laden monstrosities as their successors."""

This is not the case we refer to here. The Netscape got by version 4 had gotten an ungodly mess (and even before that), not a "small, elegant" system. And Mozilla/Firefox, the rewrite, is cleaner and more elegant than Netscape was.

Consider a 100 line Python script. People can rewrite it from scratch in 100 different ways, while improving upon it with no problem. At some point of complexity this stops being true, but Brooks was talking about huge projects, built by enormous teams, like OS/360 and such. Not some 20K - 100K line web project.