The reality is that the 2017 season is about the Yankees finding out what they have; that way they can figure out in 2018 and 2019 what they will need the most. But that means they have to cycle through Cessa/Green/Mitchell/Severino and give them major league starts so they can figure out who to keep. If they go through all of them and it's not working out, then they look at Enns or Adams or whoever else is pitching well at AAA and they give them a shot.
The Yankees aren't going to be potentially serious contenders again until 2018 or (most likely more realistically) 2019 at the earliest, and it's at that time they can go after pitching. As Cashman would say, make a "finishing move" and trade prospect for a Chris Sale type. 2017 is not the time to do that.
The entire 2016 Cubs rotation was all brought in from the outside, nobody came up through their system:
Lester - signed as free agent
Lackey - signed as free agent
Hammel - signed as free agent
Arrieta - traded from Orioles
Hendricks - traded from Rangers (though did spend a season and a half in the Cubs minor league system)
The Cubs are the template, figure out the core position players and once that unit is solid go out and buy the pitching necessary. If the Yankees develop a starter or two, great, if not they will have to go out and buy what they need (like the 2016 Cubs rotation, look at the 2017 Red Sox rotation and how all they were acquired, again all through trades and free agency). The Yankees are a year or two away from that position, just need to be patient right now.