This is really bad news for Williams. To my mind, their failure to capitalise on the works BMW engine was entirely down to their inability to hold on to their top-line staff. Ever since Newey left (which was, in my opinion, inevitable and unpreventable), they have had huge flux in their aero and chassis departments, and have lost a number of key people. This has been compounded by their bringing a new wind tunnel on line at the same time as lacking a properly-bedded-in chief aerodynamicist.
Looks like a couple of years in the wilderness for them. They just have to hang on to this guy through their engine transitions, so that they have at least some consistency.