Why would AMD, Sony or MS want to negotiate the prices of shrinks at the start of a gen? AMD isn’t a fab. It would only have an inkling of what a 2016 16 nm finfet based chip would cost pre 2013.
You don't need an initial monetary cost or value (dealing with future needs) on negotiating certain aspects of contracts, as seen in many perpetual contracts (service vendors, software vendors, and yes, hardware vendors). Many contracts have open-ended clauses and considerations towards these types of scenarios (i.e., future cost).