Wizard's intended for some PC options to be better/worse/different, when choosing different classes, and to have that difference reflected at level 1 with a difference of +1 on a primary attribute.
There is no expectation of a 16, because there is no limiting of Class and Race options. You may choose whatever you like. Furthermore, there are rules straight out of the PHB which allow for different mechanism of Attribute generation.
The 'fundamental math' of the game cannot possible have an expectation of 16, there is too much room to change one's attributes for that to be the case.
(EDIT: Oh and if they DID intend for a min of 16, then they really DID intend for races to be fundamentally tied to specific classes and archetypes.)