What is the incentive to use bitcoin due to transaction fees?
There are two scenarios we could see as the block subsidy (inflation) halves every ~4 years:


The number of fee-paying transactions increase per block whilst median next-block fee remains low/negligible
The fee per on-chain transaction increases


We are currently seeing scenario 2 play out, where most user transactions are expected to take place off-chain. Transactions that are made on-chain are expected to be settlements of many off-chain transactions, likely collecting a large number of smaller fees within.

Both scenarios are fueled mostly by adoption of the network, and until the subsidy is gone (zero inflation), increases in coin price will help to sustain miner incentive.