One advantage of stablecoins is price stability. Merchants don't have to worry about price volatility when transacting in stablecoins. In the past and present, there have been a handful of projects proposing issuing altcoins (non-stablecoins) that will lead the future of crypto payment. What's the fate of such projects, will there ever be considered viable?
For merchants definitely stablecoins make sense because these traders and retailers want to see stability and fix amount while receiving payments only merchants or traders that have good insight about crypto accept or want to accept cryptos because they know that in future the value will grow and hence their profit but it is ok if the majority wants to use stablecoins, they should be provided with that option if that helps adoption.