A few people have alluded to it without completely following through on the point. Williams downfall started when they failed to line up a stale, long term engine partner (I’m not going to start saying powertrain until next season :p).
Having had successful relationships with Honda, Renault and then to a lesser extent BMW, Williams were left in the cold at the end of 2005 when BMW decided to take over Sauber and go it alone. Part of their inability to find a suitable tie up thereafter is, in my opinion, down to the fact that the manufacturers started thinking in the early to mid 2000s that they would get more bang for their buck going it on their own in the sport rather than teaming up with an established constructor. And I suppose it is true: When you think of Renault world championships you don’t think of the fact that they won every world championship (bar 1) between 1992 and 1997, you think of the double double they achieved in 05 and 06.