I believe MS knew full well what they were doing when they released Vista. They knew to improve security they would have to completely re-vamp the OS as people understood it. They knew that there would be problems. But they had no choice.
That's why they've done it in installments, to lessen the impact.
I believe they knew that Vista would be the first installment of "Windows 7", heck they were talking about "Windows 7" not long after they released Vista.
Just my two cents!
Later

Ted