"Nostr will fail unless everyone runs their own relay."
Nostr isn't designed to scale in that manner because it doesn't have routing like lightning.
This claim is similar to saying Bitcoin fails if everyone isn't running a node. While it's preferable, it's by no means necessary.
It need not be perfectly decentralized, just sufficiently decentralized.
Whether that means dozens / hundreds / thousands of relays is up for debate.
