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Just because it's inconvenient for Coinbase doesn't make it legal. That's the kind of argument which goes absolutely nowhere in court. If you want your Bitcoin Cash, sending Coinbase a registered letter demanding delivery might be appropriate. If they don't pay up quickly, you're in a good position to sue.

Coinbase doesn't have to trade Bitcoin Cash, but they do have to deliver it to the customers they owe on demand. In the world of real securities, "failure to deliver" is a big deal. It is not at the broker's option.