Stablecoins

If stablecoins fully replace fiat currency as a digital currency one day in the future what will they be valued against?

If the reference value USD or EUR no longer exists how would these coins values regulate. I know this is not happening any time soon but longer term how do you see this working? Interested to here people's thoughts on this.

Also would there still be a need for different stablecoins or would a worldwide digital currency emerge.

Could we reach a stage where all countries share the same money. If everybody is using digital currency what would be the point of having different ones for each country or region.