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. But it was sealed when they had to run Cosworth engines. Between 2006 and 2011, Williams used Cosworth, Toyota and then Cosworth engines.
I don't think it's a coincidence that they lost their status as a premier team and also lost the RBS sponsorship during that spell. The Renault engine, and now the Mercedes engine, help.
But this is now team operating a much smaller budget than the teams it would like to compete with. I don't know how they can pick themselves up again. Perhaps by finding and hiring the next Adrian Newey?
It has been said that man is a rational animal. All my life I have been searching for evidence which could support this.
-- Bertrand Russell
To make an apple pie from scratch,
you must first invent the universe.
-- Carl Sagan