Regardless of the price, an exchange cannot seize customer assets by giving them 10 days notice for withdrawal. It doesn't matter if it's a separate currency - we have things resembling this in public markets, like stock splits. It doesn't matter if everyone can fork the cryptocurrency, that doesn't change the principle of a customer's asset.
What Coinbase should have done is this:
1. Stood their ground on not supporting trading for the cryptocurrency, because they are under no obligation to develop new features.
2. Implemented a system to disburse the forked currency to customers if needed.
3. Implemented an explicit trigger price for acknowledging the new currency, which would function for potential forks of currencies already supported on the platform. If the currency does not pass a specific (very low, far lower than $700) mark, they don't disburse it, but if it does, they immediately support disbursal to anyone who requests it.
The organic state of a cryptocurrency fork is that all extant holders receive a new asset, which may or may not be worth anything. A company cannot seize that asset without consideration. If the price of the currency subsequently crashes, then it crashes; that does not change the rights of a customer to their assets, which cannot be signed away with an email sent out 10 days before the event.