Well, they don't need to scale if no one uses it.
But if they do, you have a big problem because now a few businesses power the bulk of the network, and the users have no way to hot swap servers.
Discussion
I agree that is just one scenario that could happen. But is that something that will break the protocol? I don’t think so. I think this scenario will be played out somewhere in the future and for now I would be ok with that. With that scenario we already have a way better internet / communication layer than we have today.
Breaking the protocol is not the concern, it is that the protocol solves real problems for many people.
Without that it dies.
Let me be clear, nostr is not a "better internet" layer, it is the same web we already have, except it signs everything with a key.
Everything else is the same, or worse.
What is the main difference for you between what the internet is and what the web is?
I think we have to clear about how we see things (making the definitions clear).