Well yeah, if all underlying reserves lose value, the stablecoins are going to tank as well. And yes, holding all reserves in one form of value is not good.

But the eventual idea is to hold reserves (allow people to borrow using collateral) in multiple forms, like tokens tied to ownership of things like stocks, bonds, gold, etc. as well as other cryptocurrencies.

And despite what this guy says, DAI has remained pretty stable: https://coinmarketcap.com/currencies/dai/