expensive relays are not a pipe dream, because spam must be solved. I grant that most people will not want to pay (although the number of people willing to pay will get lower if the friction and price gets lower too), but for those who are willing to, expensive relays are a great solution to proving themselves to not be spambots.

other solutions are possible at the same time, involving manual approval of users or proof-of-work or soft-KYC or relays that pay high prices for machine-learning to filter spam but then they sell your data to pay for these losses and who knows what else.