Imagine that in the future Bitcoin or an other currency actually replaces the currencies we know today like EUR or USD.
What will back/define the worth of one Bitcoin/crypto?
Nothing, we decide the value of bitcoins, the market cap serves as proof that it has a value unlike fiats which are said to be representation of gold reserves. So what will happen if Bitcoin replaces fiat? The rich will no longer be as rich as they used to be, and the early adapters of bitcoin will enjoy the perks of believing in it.