Nostr isnt designed to scale at all.  Connection oriented networks dont scale very well.  But they can do a decent enough job.

Sufficiently decentralized is orthogonal to scaling and imho a red herring.  We know what scales through observation and we know what doesnt scale through observation.

Nostr was under 100 users 2 years ago.  It's done very well to get where it has, but it wont overtake the internet, and isnt designed to.  It's a very useful tool, and unlocks a lot of developer innovation and creativity.  Working as expected.