I love this idea but I dont think that the precursors are around anymore. I believe that the precursors left the galaxy to explore other galaxys and saw the potential the forerunners had and handed down to them the burden of caring for the milky way. I think when the forerunners saw humanity they saw the same potential the precursors had seen in the forerunners and so they made us the heirs of the galaxy hoping we would one day advance and take care of their galaxy after the firing of the halo array.