Funny thing is, Microsoft did almost the same thing with Windows NT. However, unlike Netscape, they had the resources to keep iterating their old junk while they worked on the totally new version.
Spolsky mentions that Microsoft was going to rewrite Word for Windows and Mac with the same code base, but someone decided because at the time Word Perfect was a better product, they should skip that idea.
http://blogs.msdn.com/b/rick_schaut/archive/2004/02/26/80193...