I considered that, and here's a few thoughts:
- The nature of human political control is such that a large diaspora would not be likely to happen unless humans change profoundly. Those in power would not want new colonies arising all over the galaxy because those might become powers some day and come back to haunt them. In fact I doubt the current situation - where anyone can buy an armed ship - would ever happen. Politicians would lose control over "their" populations way too fast and then there would be no value in being politicians.
- Human civilizations are resource-greedy and there may be a lot of interdependencies we don't understand right now. I think FD gets this more or less right with the various plagues and "Plugh Station needs medicine!" CGs. A human outpost 20,000ly away would be at much much greater risk of catastrophic - something - than one 100ly away, where help is hours not days away. Think back to the 12th/13th century on Earth where there were occasionally entire towns wiped out by raiders or pirates, or fire or famine or disease. Humans build vertically and horizontally integrated civilizations because we're more likely to survive if we do.
I would expect human "fringers" like the European settlers that colonized North America to break away and establish colonies way out, past political control. The Puritan religious fanatics that settled the East Coast of the US left England because they were so unpopular there, they looked for a place where they could go start anew. I would expect that sort of thing to be all over the galaxy of Elite, actually. Or, rather, their bones and the wreckage of their ships.. Pirates, famine, disease, hostile wildlife... It's a huge aspect of possible game-play that FD hasn't looked at, yet. "The Lost Colony" Or imagine what happens if the lost colony militarizes and the rest of human space encounters a psychopathic armed equivalent of Daesh? (and that's why in my first point above, I would expect human politicians to try to prevent that scenario)
Edit: I always liked the old "generations ship" scenario. Some crazy cult pools everything they can get their hands on and launches off to find a new eden. And when they get 1000ly out, they discover nobody thought to bring a can opener and they all die.