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?
I will start by saying something towards the title; any coin apart from Bitcoin is known as an altcoin that is to say, stablecoins are still altcoins, but a different type of altcoin though. Nevertheless, you are right about stablecoins, by using them for whatsoever venture, there won't be any need to worry about volatility but the only challenges might be the issue of centralization, although it depends on the view of the project aiming to use it. Also, I think every project knows what serves them best, while some will prefer stablecoins others might prefer ETH or Bitcoin, thus the choice of crypto used for payment will depend on the project itself.