If they got hacked and a bunch of stolen coins were dumped on the open market then the exchange value of Bitcoin would be the first worry, but the second worry would be the push for the seizure of any wallets with coins that could be traced back to the hack. A fork might even be possible and if Coinbase was the party at fault, they might make the original Bitcoin "Bitcoin Classic" or delist it altogether. I'd hope that the global adoption of Bitcoin would shield against arbitrary attempts to change the chain so a fork like this is unlikely from my personal perspective.