The mining reward consists of the sum of the current block subsidy PLUS the transaction fees of all the transactions included in the "mined" block. Therefore, there will never be "no reward incentive" since the transaction fees will remain as the reward incentive indefinitely. Over the next 150 years as the subsidy shrinks and thee fees grow, the fees will become a larger and larger percentage of the reward until eventually they are 100% of the reward.
To expand on this, there is
statistics to show the sheer amount of transaction fees currently being paid. With a current block reward of 6.25BTC and transaction fees averaging around 150 BTC per day, the transaction fees are already worth about 1/6 of the block reward (based on 144 blocks in one day = 900 BTC). That is already more than enough to incentivise miners and prevent a 51% attack.