I'd buy your argument if they weren't dishing out $20mil per now, at the front end. How high could his value reasonably be expected to climb? They have overpaid now, instead of waiting and seeing how he will do and then, OK, you want to keep him, you still have to overpay anyway.
I'm surprised by the timing, and don't like the money. Every time the Sox seem to do something smart, they turn round and do something that's questionable at best.