PDS is just one part of the whole thing. It’s just the place where your data is stored. Almost all other parts of the network are dependent on Bluesky’s stack. Even the alternatives like Blacksky and Eurosky are still dependent of Bluesky’s implementations like the Appview. So if Bluesky goes down, it affects the other services (in some way) too. One can host a PDS for cheap, but if you want a proper social media service (with post history, search etc), it is kind of expensive. There are other appviews, but for some reason Bluesky going down causes issues in Blacksky, Eurosky etc