Who says Savin needs to threaten William? Just because Savin isn't the main employer doesn't mean he doesn't have control over his team structure. We're talking about a 40k month game, not some small time community college theatre project where the director is forced to take on actors at the producers whim.
Like
RenH also mentioned. William could just be moved to a different team or have control to throw him somewhere besides CoC2. Project management is the fine line between the bigger bucks people and workers. Savin as the midpoint who ensures the game is made in a timely fashion with quality standards set by him or Fenoxo or the stakeholders still bears responsibility of not letting the employees run rampant or produce little to no content.
William, Tobs, etc. could drag their feet and Savin would/should be on the chopping block if he can't get them working else they need to be fired or transferred under another project manager to take care of them.