If you come up with a tool that will pick up on a diagram in a readable ascii format in the middle of a document, let me edit it, and then patch in the updated diagram without making any kind of changes to other portions of the file, then we'd be getting somewhere.
Even then there'd be issues, as e.g. I might want to modify descriptions elsewhere in the file in between modifying the diagram. As a concrete example, I am doing that these days with a spec for a system I'm planning. I'm using a custom Markdown based processor with a filter that takes Graphviz/dot syntax inline, and while I edit the diagram, I'll also then often want to write something about what I've added to it. So if I was going to use an external program, the roundtrip from text editor -> diagram editor -> text editor would need to be very fast and smooth.
Though to be really useful for me, it'd need to work for me via an ssh connection as well...
It's really hard to beat plain text for some of these use-cases. At least without first getting more graphics capabilities back into our terminals.
