@darrenj It wasn't exactly the Federation's fault though, especially given how it was not an experiment like SB19, unless you are referring to how no one responded to Su'kal's ship's distress signal pre-Burn. There is the implication that the Federation did nothing to help, but it is also never pointed out as a hindsight solution. In TNG, if important, I think they would have definitely pointed out how the ship could have been saved, which would avoid the Burn, and they would have built the episode around that and its themes.
I do like the idea that it's range isn't actually known. DIS has seriously problems with scope and scale so I doubt it will turn out to be anything less than the whole galaxy being effected, and I expect the whole thing to be forgotten next season. With a worse Burn I would hope its effects would be too great to ignore part way through the story.
I see Earth leaving as an extension of how Worf was able to leave Starfleet at will and later return without any loss of position and rank. That puts the Federation into much of an EU, UN, NATO concept than Space USA. The problem is Earth and Vulcan leaving really needed the context for why they left. The Ni'var say it was being forced to use SB19, but not just that, that it had several other unexplained reasons for leaving. We need those unexplained reasons for context, because the closest we get to context is some nebulous indications the Federation was overstretched pre-Burn, and maybe failing its obligations to founding members as per how the Andorian rebel says the Federation is infamous for not keeping promises. Exploring those reasons through the Chain could have been really interesting, and a way to characterize the Chain earlier and more permanently as more than just a fragile union of mobsters.