Lower fees than credit cards or Paypal?
Sure, credit cards and PayPal are free for me, but the merchant is often charged a few percent, sometimes as high as 5%. Bitcoin transactions cost me a few cents and the merchant nothing. Because of this, the majority of merchants I spend bitcoin at offer a discount for paying by bitcoin which outweighs any cashback or fee which may otherwise swing things in favor of fiat.
Or they have to use a company like Bitpay, who can admittedly undercut Visa or Mastercard by 0.5%-1%, but still charges 1% to the merchant.
If they are motivated, they can use a service like BTCPay which is self hosted and therefore charges zero fee. The only fee for conversion to fiat would be the exchange fee.
Block size, blockchain bloat. Aren't Lamport signatures gigantic? Not a total deal breaker but something we hope to delay for as long as possible I'm sure, as it will seriously damage decentralization.
Lamport signatures are larger, yes, but they aren't the only option. And even then, if you are only using them to open or close Lightning channels, then it is not such a big deal.