> Data flow and perhaps control systems (AI scripts and GUIs built around something declarative and event-driven come to mind) indeed seem to be the only commercially successful examples of visual programming thus far.

I strongly disagree. Here's an incomplete list of commercial software in VFX for visual "programming" for artists:

Houdini by SideFX

Katana and Nuke by The Foundry

Massive http://www.massivesoftware.com/massiveprime.html

Gaffer https://github.com/GafferHQ/gaffer