It's the same when currencies around the world stopped using the gold standard and became floating across each other.
I'd say the value of currencies is that you have to pay your taxes in one that is legal tender. In that case, the value of Eth is that you have to pay transaction fees in Eth. You decided how valuable those transactions are.
Governments have a monopoly on issuing fiat currencies, but for PoS cryptocurrencies there is no enforced scarcity—any DApp that can run on Ethereum can easily run on any of an infinite number of Ethereum clones, so why bother using the expensive congested network?
When you sell your Bitcoin for Ethereum you have to pay taxes in dollars. The specific value is dependent on the ability to exchange it for goods and services but the fact that it will remain valuable (but not how valuable) in the future is secured by taxation. Who knows what is going to happen with Bitcoin? It may get replaced by Ethereum entirely.
I'd say the value of currencies is that you have to pay your taxes in one that is legal tender. In that case, the value of Eth is that you have to pay transaction fees in Eth. You decided how valuable those transactions are.