Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
So the seller can accept stable coins if he doesn't want to see the bitcoin price is volatile so after he gets his stable coin, he can hold the stable coin, or he can send it to the exchanges to buy bitcoin. The seller will have so many options for the coins that he wants to use as the payment system, and he doesn't have to used bitcoin if he worries with the fluctuated price of bitcoin.
Bitcoin price will be like that, while the seller can allow their customer to use cryptocurrency to pay whatever they buy from the seller. That can help cryptocurrency grows too so many people can know that they have another option to pay.
I think there is a seller who can accept the risk of the fluctuating of bitcoin price so they can still use bitcoin as the payment method.