If the restaurant uses the stablecoin, nobody takes the loss because the transaction is happening with a different currency. The restaurant isn't holding BTC,

If the restaurant uses the stablecoin, nobody takes the loss because the transaction is happening with a different currency. The restaurant isn't holding BTC, it's holding a USD-equivalent. If BTC loses 10 percent of its value overnight, the people who lose are the people who sold at the lower prices. When we say "the price of BTC is X", what we really mean is that a trade happened at that price. That means that someone thought selling at that price was the right thing for them to do and someone else thought that buying at that price was the right thing to do.