Sovereign domains aside, I'm betting on using nostr relays simply as an ephemeral storage that keeps nostr events persisted for free for 1-3 days tops. This happen to pair well with the fact that most microblogging posts aren't meant to be revisited too far after publishing date.
The source of truth becomes the user devices' local DBs that gets synced (syncing uses nostr relays too).
This could make Nostr its own thing that doesn't need to compete on the same terms of players with deep pockets.