We were trying to analyze communication networks, not conventions. As you noticed, Cleveland cannot hold large conventions because it doesn't have a fundamental requirement for that (a good taxi supply). If you want to draw analogies, that's something like saying that a network that isn't working can't be used.
As to the upper boundary question, there isn't really one. Even if there is only one game in town, the value of the network affects how much people will be willing to pay for it, and how much usage of the network there will be. In other words even once network effects have forced everyone to one standard, there are consequences of the size of network effects.
On why we don't have graphs, that paper is supposed to be headed for an academic publication. It may not be understandable for most people, but it isn't supposed to be either. (Of course I don't think that most people here quite count as most people...)
Cheers,
Ben