Default stroke check box next to stroke weight
Please, please make a "Default" checkbox next to the stroke weight that can force all future strokes to the set weight when checked
There is another way of doing this (kind of).
Create an rectangle or whatever, open Graphic Styles, apply the default black/white one to your shape, set the stroke weight you want, than Alt+drag this shape over the default style.
And every time you press D you’ll get this particular weight.