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?
Users can still use other stable coins in different network layers.
Almost all other stablecoins are junk or extremely risky.