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.
This may be an unpopular opinion, but nobody really cares what merchants want. Merchants need to support the payment options that consumers want and use, not the other way around.
Credit card companies literally pay me to use credit cards. It costs me nothing, and the lack of instant settlement only protects me as a consumer. Why would I prefer to use BTC if it means paying fees, waiting for settlement, and having no consumer protections? Because it's better for merchants?
If merchants can save so much by avoiding credit cards, they ought to be giving sizeable discounts for BTC payments. I don't see why consumers would be interested otherwise, outside of the phenomenon of spending BTC as a means of realizing capital gains and avoiding income taxes.
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.
What other options are there that are widely considered safe and adequately tested? Why would this be limited to the opening and closing of Lightning channels?