Just thinking aloud to myself. Here are my assumptions:
* Bitcoin price is backed by mining electricity cost
* Speculators cause wild fluctuations around this price
* The actual usefulness* of bitcoin as a payment system adds extra value increasing the price
So, if you take away (or at least reduce the massively) the mining cost, what you're left with is the actual usefulness* of the currency backing the price and speculation around that value.
Therefore, POS (and by association DPOS) systems are valued almost solely on the usefulness of the currency.
* Usefulness of a currency I'm broadly defining as the sum of all things you can do with it, like paying for bills, products, keeping your money safe, etc etc