What does it mean that they will be only used for payments? I know that almost all those stable coins are mainly used as a trading instrument. This is can explained how the first USDT stable coin was made by a trading platform then expanded yo other networks. Means that USDT at the time weren't used outside of those platforms before supported by known networks.
I didn't read anywhere that it will be only used for payments, but that payments will me one of the main uses. Where did you see that?
First stablecoin ever ussued was back to 2014 called bitusd in the bitshares blockchain network. This initiative was encouraged by exchange platforms as an additional feature for its users to freeze their crypto balances when price crashes. This is why i called it a trading instrument more than a real cryptocurrencies especially those backed by the US dollar.