Oh yes I'm completely with you on wishing Github would support PlantUML. It's almost trivial implementation wise, it's highly embeddable.

It's precisely because I do like it that I want it to scale up better to more complex diagrams. I basically can't push it forward as a universal standard in my org because it fails above a certain complexity threshold and I really can't push for teaching everybody a thousand ways to tweak the diagrams to coax it into doing the right thing (and even then, one small change and it may completely rearrange the output).