Jason Heyward will turn 27 years old in August of 2016, a season after putting up a bWAR of 6.5. He'll also turn 28 years old in August of 2017 and 29 years old in August of 2018. That's how time works. At each of these time intervals, Heyward will be younger and probably better than any free agent hitter available to the Yankees. Let's look at his competition: After the 2016 season:
Josh Reddick (Age 30, 3.5 bWAR in 2015) Colby Rasmus (Age 30, 2.6 bWAR in 2015) Carlos Gomez (Age 31, 2.3 bWAR in 2015) Justin Turner (Age 32, 3.9 bWAR in 2015) Neil Walker (Age 31, 2.4 bWAR in 2015) Jose Bautista (Age 36, 5.1 bWAR in 2015) Adrian Beltre (Age 39, 5.8 bWAR in 2015)
After the 2017 season:
Brett Lawrie (Age 28, 1.9 bWAR in 2015) Eric Hosmer (Age 28, 3.6 bWAR in 2015) Mike Moustakas (Age 29, 4.4 bWAR in 2015) Lonnie Chisenhall (Age 29, 2.3 bWAR in 2015) JD Martinez (Age 30, 5.0 bWAR in 2015) Brandon Belt (Age 30, 3.9 bWAR in 2015) JD Martinez (Age 30, 5.0 bWAR in 2015) Brandon Belt (Age 30, 3.9 bWAR in 2015) Cameron Maybin (Age 31, 0.6 bWAR in 2015) Lucas Duda (Age 32, 3.0 bWAR in 2015) Lorenzo Cain (Age 32, 7.2 bWAR in 2015)
If Jason Heyward were a free agent after the 2016 or 2017 season, he would almost surely be the most valuable hitter on the market, even if every single listed hitter above makes it to free agency without signing a contract extension That means that even if the Yankees decide that they want to open up the checkbook after Mark Teixeira, Carlos Beltran, etc come off the books, they won't have as good of an opportunity as they have now to improve the team.
Here is the scenario that I am hoping for: Cashman has permission to go out and sign Jason Heyward, but only if he can clear equal room on the payroll. Cashman is trying to trade both Brett Gardner ($13.5m) and Andrew Miller ($9m) in order to offer Heyward something like 10 years, $225m (Conveniently [13.5 + 9]*10, right at the AAV predicted by the Fangraphs crowdsource)
The team would almost certainly get better from those combinations of moves. Not only was Heyward worth more wins in 2015 than Miller and Gardner combined (6.5 vs. 5.5), but the Yankees would likely get something back in the trades to improve the team. Even if the returns are all prospects, the Yankees could use the extra roster spot to add a win or two pretty easily.