Well, might as well start this, as Kaby Lake is due out in 2H of 2016, and is well placed to hold out until 2H 2017 when the new architecture of Cannonlake is released.
To me Apple will hold off updating the Retina MacBook until Kaby Lake to take advantage of TB3 being enabled on the SOC, which makes sense for such a diminutive notebook. As Skylake without Intel`s Alpine Ridge is merely a spec "bump" Nor is the rMB in dire need of a meaningful update unlike the rMBP, especially the 15".
I wouldn't use the Mac Pro as an example of Apple's typical upgrade cycle. If you only consider Apple's laptop upgrade cycle history (warranted in this case), the MacBook is all but guaranteed to get Skylake.