Yeah this is just the thing - you cannot stop the bitcoin blockchain because it's decentralized, and due to competition, individual exchanges can't call for a stop on trading. They can but people will revoke trust in that exchange and move to another one. If Coinbase is down more often in times like this, they will start losing trust too and people will move to the next best competitor. Then again I think by now Coinbase has built up such a good reputation and whatnot that it'd take a lot for that to happen.