Originally Posted by wxwax
It's hard to pin Williams' decline on the engine, but it's tempting to do so. Their disastrous relationship with BMW is when it started.
Their decline began when the rule change banned their active cars. When Newey left them for McLaren it didn't help (it didn't help McLaren much either - Newey worked best when he was under the hand of Patrick Head - at least until he moved to Red Bull).
The Mechachrome era was just Renault with another name, the BMW era saw Williams supplied with arguably the best engines in F1 at that time. The problem with the BMW era is that Williams and BMW didn't have the same intentions: Williams just wanted an engine supplier but got a "partner". BMW wanted their own team and thought that a creeping take-over would work.
I've come to the conclusion that the real reason for Williams F1's decline has been Frank Williams attitude to his drivers. It cannot be very motivational to be considered little more than "just another component". Frank's single-mindedness has contributed to his longevity in circumstances that have bucked the medical prognosis but it's also contributed to Williams F1's decline.
For a long time now, sad to say, the problem with Williams F1 has been Frank Williams.