I think you're not understanding. While a bunch of people losing money is bad, it wouldn't create another Great Depression, per se. A huge investment bank with millions of counterparties going under absolutely could cause massive, long term damage to both the financial markets and the real economy. For example, people losing their hats in crypto wouldn't cause commercial paper markets to dry up. A big IB going down almost did in 2008. That would have resulted in huge businesses that employed, collectively, millions of people becoming effectively insolvent. Now you're talking about a Great Depression. Big difference. This is where the keyword "systemic" comes into play–as in, this entity is a critical node in the system. Its collapse could jeopardize the system.