For now we need not worry about transaction fees being the sole supplier of miner revenue. This was a brilliant design by Nakamoto to make the last bitcoins be mined in at least 100 years.
But there is a self-feeding negative feedback mechanism here. If the miners make fees too high, the utility of bitcoin diminishes to unusable levels. If that happens, the price depreciates significantly, and therefore miners' revenue falls even more than the money they're making off transaction fees. It's an optimization problem, and at the moment they have hit that optimization point.
Our job is to change the optimization point to a much lower transaction fee. It's kind of similar to Mars' atmosphere. It is currently at an equilibrium pressure (I can show you the graph if you want to see it, just ask), which also happens to be the triple-point of water. However, if the temperature of the surface of Mars is increased even 10 degrees Celsius through human intervention/ingenuity, the planet achieves a new equilibrium point of 50 millibars surface pressure (enough for humans to survive without pressure suits), the point at which it will stay nearly indefinitely.
Miners are optimizing their revenue in much the same way, but if the community puts pressure on them, we can "force push" that equilibrium point back to where it should be, a much more farsighted vision of bitcoin where micro-transactions are possible, cross-planet transactions are possible, scaling is possible, and much more.