CONFIRMED BY BEZOS: Amazon 3P sales accounted for 50% of units sold worldwide in 2017.

I know we assumed sales were moving this way, and many reports indicated this shift, but always validating to hear it direct from the top.

In the beginning, I believe Amazon wanted two things through their retail channel (ie Vendor Central):
1) Selection 
2) Low Prices

By connecting directly with brands & setting them up within Vendor Central, they very quickly attained these two things. By having control of the inventory, they have control of the pricing (regardless of profitability). 

I believe their ability to control pricing (and keep it low), was the instrumental tactic behind becoming recognized as the low price leader they are today.

However, this low price tactic has frayed relationships with some brands. This is a driving force toward the 3p sales shift in my opinion. Brands now sell more direct through 3p, or are working with one or a few preferred partners to execute the strategy. 

Will be interesting to see what Amazon releases on the Vendor Central side to stay relevant.
-Will they now recognize MAP agreements?
-Will they accept cost increases in a timely fashion?
-Instead of CRaP'ing out on an item, will they just raise its price?

​We shall see!