HN (not YC, who readily invest in blockchain companies) are usually about a decade out regarding blockchain knowledge. Paying 2-6% of all your transactions to intermediaries of varying value-add may seem sensible to you. That's fine.
Merchants aren't the customer target for credit cards, consumers are. Credit card payments are reversible and provide a reward. There are lots of options available that are better for merchants than credit cards (cash, debit cards, transfers, etc). But they all lose because the consumer prefers credit cards.
Yes, that's the varying value-add mentioned in the comment you're replying to. I pay 3.5% of every card transaction to Square. I don't get 3.5% cash/rewards back.
Do you get a discount for paying with cash (or blockchain)? In general the answer is no, meaning you aren't paying the 3.5% transaction fee, the merchant is.
Cash isn't really great for merchants. You have to handle it, safeguard it, count it, get it to the bank. Many hands are involved that process and theft or loss can occur by any of them or by robbery/burglary. I don't know if it's a break-even on payment card fees but I bet it is close.