The important bit is that it's optional. We need to accept proof of work as verification that you're not spamming. We want to build an accessible system, not fences.
Discussion
I somewhat agree. Truly public relays, in the fashion that exists now, cannot survive. They'll get buried under spam, porn, and csam and be destroyed by it all.
Public relays that only provide write access for those that prove work however could be viable for those who just want to be on nostr, and will accept shitty service. You could pull that off by connecting your pubkey to a UTXO via on-chain address signature, or a recognition signature by a trusted party. It would stop the spam, but it wouldn't fund the relays, and they have costs. A charity relay that funds itself as a goodwill gesture is one thing, but demanding that of a relay is as immoral as demanding people zap you.
The only way paid relays become a walled garden is if there aren't enough of them. Public relays provide garbage service. The moment I went fully to paid, performance and reliability went way up. Most people will pay for good service that they value. If there are only a couple dozen paid relays, then all the good service is centralized. If there are thousands with good rates, then most people can connect to 20+ without a dip in quality, and the chances that people end up with nearly identical relay lists approaches zero.