Someone created a text version of the PDF on StackerNews: https://stacker.news/items/1279868

Relays have no incentive to cooperate. You cannot pay "relays" collectively, you can only pay a specific relay. And other relays have no way to tell that the payment was real. With proof-of-burn, you can pay the Bitcoin miners collectively, and anyone can verify the proof.

You do a lightning transaction for each upvote. The on-chain transaction is done by the notary and it can aggregate millions of posts. It may also be possible to pay the notary using cashu, or whatever they decide to accept.

The Price of Attention: Attaching Bitcoin Fees to Nostr Events https://github.com/spesmilo/notary/blob/master/notary.pdf