Today, Coinbase is holding over $100 billion in Bitcoin.
You got source they are hodling his huge amount of bitcoin?$134.5B :
https://river.com/learn/who-owns-the-most-bitcoin/
Coinbase has something like $120B in Bitcoin keys. That's like having no coverage at all, basically 
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.They might not be completely ruined but it would destroy their reputation and credibility. They still have a lot of altcoins and fiat, which could cover some of the losses but it would take a long time to fully restore customers’ balances.
I agree it's pretty unlikely, but... they would be completely ruined if they lost $120B in customer assets. There's no way they would ever be able to restore even a fraction of customer balances if that happened.