I don't think that the complexity issue (...) is essential. I think it's self-imposed by current visual language designers. Perhaps that's because the handful of visual languages that got traction (and thus the benefit of countless hours of design & development effort) are created for people who aren't computer scientists. These languages only need to be sufficient for solving a domain problem, not for advancing the art of the visual PL.
The fact that current visual languages are built of monochromatic text, a few box shapes, and a few colors of line, is a tremendous failure of imagination.