It's extremely important to remember that when the fees make up the vast majority of the miner's income, there will be far more people using bitcoin. Ultimately, if bitcoin fulfils it's promise as the best store of value mankind has ever seen, everyone in the world will want some. In this situation, the demand for the 7 tn/sec will be enormous. As humans we're just not used to seeing hard limits on supply of a liquid asset, and it's easy to overlook it's effects. Just as the hard limit on the bitcoin supply issuance is fundamental to it holding its value against essentially anything else (even the gold supply doubles every 30-50 years) and will lead to enormous growth in demand against a falling supply, the fixed supply of transactions will lead to similar increases in the price of transactions due to fixed transaction supply and increasing overall demand.
Once hundred's of millions to billions of people are fighting over transaction space that can only service 150 million transactions a year, the supply/demand ratio will be plenty to support a high price. By that point people would _ideally_ be using it every day/week, and so the potential demand would be enormous, and the transaction price will increase until only their larger transactions are economical.
The real question is: how much hash-rate is really needed?