What are the biggest unsolved challenges nostr will face before it's massively successful and serving billions of people?

Some of these might be unresolved issues and require new takes on the problems since they have not been solved before.

How to measure the true level of decentralisation?
For example, all relays serve less than 1k users

How to ensure that centralisation is avoided by a better design?

I'm asking questions and don't know the answers. But as Bluesky seems to have failed to provide the path forward there aren't too many viable candidates left for a decentralised social and Internet future.