I only brought up halvings because it explained the chart. But let's ignore halvings and talk about mining incentive then. Fees are the less important half of the story. The main driver is difficulty adjustments. If scarce fees come in, difficulty goes down so profitability increases. If plentiful fees come in, difficulty will go up so profitability decreases. Fees are actually damped by the adjustments so the absolute amount of fees doesn't really matter. So long as fees are above 0.00000000, the network will be fine. I predict that there will always be some baseline level of competition keeping fees above 0.00000000, but if that is wrong, then things would likely fall apart one day.