When running a full relay gets too expensive, all apps will rely on two or three of them. When you get censored from those few (or when your constantly hot key is compromised) no one really knows the authoritative source of your data anymore. This is known as "discovery", a feature Pubky has, but Nostr does not. Pubky is also scalable, because apps can utilize indexers to aggregate all of the source data from everyone's personal homeservers.