You will probably will get the same mess. There is plenty of literature about that. I.e: A complete rewrite is what killed Netscape
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). Hardly "plenty of literature about that".
Actually, the complete rewrite might killed Netscape, but it saved Mozilla and Firefox. And Netscape was a multi-million line web browser engine, with a javascript interpreter, a full mail client and a wysiwyg editor thrown in. And multi-platform in C/C++ to boot.
That is, something an order of magnitude more difficult than LinkedIn or 99% of web properties out there.
There have been TONS of successful rewrites. Especially in the web space, it's almost trivial to rewrite your webapp or parts of it. To name but a few:
Twitter, the new Digg, SoundCloud, Basecamp, etc etc.
"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.
What do you mean?
We wouldn't have those if it wasn't for the rewrite. The old code was a mess even before version 4 (by its developers own admission), and it could never get to the point of competing in the engine space ever again.
That is, with the old Netscape rendering engine it would not be possible to extend it to compete in the modern HTML5/Canvas/GPU acceleration/CSS3/add-ons/separate contexts for each tab/etc era.