A social graph is more than just data, it is trust, it is attention, and it is political goodwill. That's one of the most important lessons of the fall of Google Reader. It wasn't just that the technology was shutdown, but that it damaged communities when it shutdown. You can't capture communities that don't feel like they belong anymore, that no longer trust you because you closed their former town square.
I still don't see enough evidence if BlueSky itself shutdown that ATProto survives that shock culturally, even if it is built to do that technically. If Mastodon.Social, still the biggest instance, shutdown tomorrow a number of Mastodon instances wouldn't even notice and some of the ones that did notice would just as likely throw a party as lament the disappearance. That's a pretty big cultural difference, not a technical difference.
When developers feel empowered to revive products or fork them, I think that eventually seeps into communities. That it’s another way of doing things. It doesn’t happen overnight but this energy exists in the Atmosphere. Maybe loss of Bluesky is not survivable culturally at the moment, but maybe it will be in a few years as Atmosphere grows and matures.