Earlier quoted context omitted.
Yeah they wouldn't just toss away millions of lines of kernel code.
I recall reading somewhere (can't find it for beans now) that the Windows kernel was a huge mess codewise since many of the original developers have moved on and poor to nonexistent documentation was left, meaning that the modern employees had to basically reverse engineer a lot of the code. Take this with a grain of salt, of course.
http://www.kuro5hin.org/story/2004/2/15/71552/7795
I'd say, the user experience has improved since then overall (meaning the code quality, too, in one of the most practical senses).