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?
What are the projects? Would they solve the problem of high volatility or something? And how do they plan to solve it?
Well decentralized/good stablecoin backed by safe and more stable fiat currencies should be more ok for "merchant crypto payments". They are better for merchant crypto payments though while the established cryptos like Bitcoin are better for long-term store of value and ok for merchants payments.
I think I would combine both for payments until we eventually have a better deflationary crypto with moderate volatility.