Please bear in mind the counter article everyone is citing (http://www.joelonsoftware.com/articles/fog0000000069.html) was written 11 years ago. I'm not saying that it's wrong because it's from 11 years ago, but you really want to consider its central example.

Netscape 6.0 was a complete rewrite. The article claims that this was a mistake.

Now, if you would recall, one of the reasons Netscape 6.0 was completely rewritten was to use Raptor/NGLayout. Raptor/NGLayout was later renamed to Gecko. Gecko is the currently developed Firefox rendering engine, and Netscape 6.0 marked the start of the Mozilla foundation.

I would harshly argue that Gecko would not have taken off nearly as well without Netscape's rewrite. But this is definitely not a story about Gecko's popularity. It's about the reason why Gecko was the decision they took at the time, and the current impact of Gecko.

Besides the fact that Gecko was one of the early open-source success stories, the reason why Netscape decided to scrap everything and start from scratch was simple: Netscape 4.0 was horrid. Remember back to the days of IE vs Netscape. Netscape was significantly slower, and had no dynamic HTML among many other features. It was buggy and would crash frequently. And the development on the "continuation" of 4.0 code was simply too slow to compete, because of outdated, not modulated code, and stuff stuck in that simply "worked," and never changed to allow for future changes.

Now, people attribute Netscape 6 to Netscape's death because of this radical decision. But take a look at Netscape's usage statistics: http://upload.wikimedia.org/wikipedia/en/1/16/Netscape_Navig... . Hey, guess when Netscape 4.0 came out and IE started to dominate? 1996-1997. By 2000, when Netscape 6 was released, Netscape's usage already dwindled down to nothing.

Now, you're thinking, why didn't Gecko save Netscape? It takes too long to reverse these tides. Netscape was already dead. AOL acquired Netscape in 1998, to combat IE. Then, in 2003, AOL won an antitrust suit against Microsoft, and allowed AOL to use IE royalty-free. Netscape was pretty much scrapped at that moment because the damage had been done.

But wait.

Mozilla released Phoenix in 2004, based on Gecko. You should now the story from there. (Clue: Phoenix -> Firefox)

Disclaimer: I'm not saying that Firefox > IE or Chrome or whatever. I'm saying that the codebase Netscape 4.0 was based on was doomed to fail. Gecko still lives on today, and was started as a rewrite of Netscape.

Anyway, my point is, there are times to rewrite things, and there are times to keep them the same. If you have a good, legitimate reason to rewrite, as Netscape or NowJS did, you should. If you are just the stubborn programmer that always implements his own Queue or QuickSort when libraries exist, don't.