Then after XP they pinned the needle the other way, and a rewrite orgy began. It produced some good things. XAML is good. They completely rewrote Window's TCP/IP stack; which is quite an act of chutzpah, and now it supports multi-core CPUs in useful ways. But overall they ended up suffering exactly the sort of problems that a Big Bang approach would bring.
In a way it's a smart move though. Most people are quite happy with XP. Any next version had to be a long-term, next-generation thing. People will love Vista in 2009.