The main reason Nostr is "decentralized" right now is because hardly anybody uses it.
If it were to scale up to millions of people, either everybody would be on the same few centralized relays (goodbye decentralization and censhorship resistance, the entire reason Nostr even exists), or people would be fragmented into a multitude of different networks and not see each other.
Nostr really needs to sit and think about this one fundamental thing it's currently completely failing at.