The fee incentive is required if, like Bitcoin, you have a capped money supply (and, therefore, the block reward must decrease over time). A key insight into N

The fee incentive is required if, like Bitcoin, you have a capped money supply (and, therefore, the block reward must decrease over time). A key insight into Nakamoto Consensus is that the security model depends on the the economic rationality of mining honestly: the difficulty of executing a double spend increases exponentially with the number of blocks the victim waits between the transaction hitting the chain and them "accepting" it, but the reward for executing that attack scales linearly with number of blocks (double spend + block reward/fees), which means that the return on investment for the attack decreases over time. The return on investment for mining honestly instead is constant: it's just given by your share of the mining power. Assuming a worst case scenario of an attacker having just shy of 50% of the mining power, I can calculate how many blocks I should wait before the money I received is considered secure, and that time grows as the amount sent grows in respect to the block reward. Without fees, the block reward goes to zero over time, and the time-until-secure goes to infinity.