DAI and SKY are backed in large part by USDC, so they are not truly decentralized. It is possible in theory, but nobody has successfully done it so far.
It's possible in practice: that's how DAI worked originally. It's just not very competitive where the main customer -- traders -- want a lot of liquidity and razor thin spread.
DAI made some dumb decisions for market reasons recently but it was an actual stablecoin for a long time. It worked fine, they just decided to make it worse for some reason.
