But that's not how the decentralized ones work. They aren't relying on "a rapidly rising market cap". Most of the stablecoins are backed by volatile assets (ETH

But that's not how the decentralized ones work. They aren't relying on "a rapidly rising market cap". Most of the stablecoins are backed by volatile assets (ETH, BTC, etc), but have automated systems to liquidate the collateral when price drops occur. The only real problem would be a very large drop in a very short period of time, and the big stablecoins already have already survived 50% drops in pretty short time spans.