They have said over and over (maybe not in these words) that they think it's essential for apps to have a "guarantee" that they have seen all -- i.e. that users have read all that was written to them, or by someone, or with some tag etc -- that implies that the entire ecosystem of apps should rely on a single "relay". Their architecture diagrams also depict all the pieces talking to a single relay, their codebases all point to a single relay. There is no provision anywhere for an app that uses two or more relays. Of course they will say it is possible for others to run alternative relays, and they want that only insofar as it will validate their idea of decentralization, but the truth is that if someone is running an alternative relay they will not be able to use any of existing online infrastructure -- they will have to run alternative versions of everything configured to point to the new relay URL. If someone is running a relay today it's probably for hobby purposes, maybe not indexing the full network, someone trying to make their closed little sandbox of atproto, I don't know, a serious relay is already hard to run today, should get increasingly harder and virtually impossibly hard if Bluesky achieves its world domination plans.