The problem boils down to this, suppose total bitcoins where worth 1 billion dollars, an average day exchanged 1/10th of that or 100 million dollars worth of transactions. Now suppose a 1% fee per transaction that's 1 million dollars worth of computing power per day or ~41k per hour (assuming mostly rational actors). Now suppose it costs 10x that or 410k per hour to take over the network. Well the 'pot' is worth 1 billion so that's a clear win and someone would do that.