He's already had the "sentimentality (overpaying) contract." This negotiation should be all about business.
You sure about that? Hindsight is nice, but when the extension was signed (February 2012), he was a 27-year-old 3B who had hit .296/.370/.869 over the past three seasons while making an all-star game, winning two Silver Sluggers and a Gold Glove, and getting MVP votes twice.
For comparison, Rendon at the same age was coming off a three-season period of .280/.368/.829 and had missed more games due to injury in those three seasons. In other words, at age 27, very similar players. Wouldn't you have liked the Nats to have signed Rendon to a 6-year extension two years ago? Obviously things diverged from there, but let's not try to revise history.