Today, most stablecoins track the value of fiat currencies, but we could see the adoption of coins tied to valuable assets like gold. These could, in fact, turn out to be the best for "paying with crypto" due to their resistance to inflation. But for daily transactions, whether they use BTC, ETH, SOL, etc. – transaction and conversion costs will be significant. The cost to pay for goods and services must be less than the average 3% transaction fee charged by major credit card, debit card, and app payment processors to be valuable to merchants.