I think that a big part of the fight over Sandoval is that even if his decline over the last few years was minimal before last year, he hasn't been very good for even longer than that. He was a 2-3 win player since 2011, before being a -2 win player last year (by fWAR). Even if you are assuming a bounce back year (and I am), it's about what he is likely to bounce back to, since the trend over the last 3-5 years suggests he won't be bouncing back to the 3-5 win player he once was. I think that even with a moderate bounce, he is most likely going to end up in the 0-2 fWAR range, which is still a big improvement over last year, but not an especially valuable player in a broader context and certainly not worth his contract, so I think that there is room to think he will bounce back and still not earn his money in a $/WAR sense.
Of course, between the no context and huge context is the Red Sox context, in which, he is likely to provide more value at 3B than the next available guy on the roster regardless of cost, and there is no FA 3B who is likely to provide more value at any price, so we would need to trade for one, and it's likely that the cost in prospect value would more than outweigh the difference between Panda's likely performance and contract. If he ends up being a slightly overpaid 1.5-2 WAR player next season, then he certainly won't be to blame if the Sox fail to make the playoffs again.
edit: should have looked first, but Steamer projects 1.8 WAR, which would be fine with me.