Cryptocurrencies slow down the rate of data not because of spam but because a slower rate means a higher consistency across the network: cryptocurrencies' goal

Cryptocurrencies slow down the rate of data not because of spam but because a slower rate means a higher consistency across the network: cryptocurrencies' goal is to agree on a consistent state with peers who do not want to negotiate. If the consistent state is pure garbage then that is not a problem for blockchains, because from blockchains' point of view, everything is fine. Spam is not a function of rate but of content. Spam can absolutely be sent in a blockchain, as you say, and making the price higher only makes both spam and non-spam more difficult. Spam for me might be actual legit information for you. Hashcash is another beast, it only has the proof-of-work part, not the money part (contrary to its name) so it's not comparable.