Well there are only 21 million bitcoins in total and when all bitcoins have been mined essentially miners will no longer recieve a block reward for validating transactions. In this case, miners will  need to rely on transaction fees in order to maintain operations. This will inevitably mean that transaction fees will have to be higher in order to continue to incentive them to maintain operations profitabily.

On the other hand one could argue that over time that mining chips will become smaller more efficient then they are today  and that therefore the mining process will become cheaper, allowing for the transaction fees to be sufficient incentive for miners to continue operations