> it is hard to identify a specific economic problem which they currently solve It is hard to identify such a problem till you believe you can withdraw your money from a bank anytime. Visiting a bank and getting an answer "please come tomorrow" for several weeks changes a person's view on cryptocurrencies and problems they could solve, imo.
Imagine trying to pay for something with crypto and having the seller deride it as fake money that they refuse to accept. That is illegal with officially minted currency.
You are absolutely allowed to put up a sign are your store that says "we only accept bottle caps as payment".
Legal tender is only regarding DEBTS. It does not force you to accept pennies at your grocery store.
You can turn down USD for payment all you like.