It’s a good question to think about actually. What happens when the 21 mil cap is reached and miners get no more block reward. What will incentivise the miners to continue mining. Will transactions fees have to compensate for the loss of block reward? In that case wouldn’t tx fees be incredibly high? So where is the utility as a currency at that point? I don’t know the answers to these. They should be another thread all together to be honest.
Yes after all bitcoin have mined, Miners will definitely depend on transaction fees as the only incentives, we have discussed that many often in this forum. Miners already are having incentives from transaction fee which are much more than the block reward itself. A thing you need to know is transaction fee are charged in bitcoin, base on current rate of bitcoin price raise the price increase might be actually help, for example after the second bitcoin halving when the reward was reduced to 12.5 bitcoin you can clearly see that the current 6.25 is almost equivalent to it in terms of the fees. So this could actually help in the future too but the problem will be, will bitcoin be ok for micro transactions due to the fees that will be paid then?
So in essence the value of tx fees (even if the amount of bitcoin for the tx fee) are going to increase to cover the lack of block reward. So if we paid a $10 tx fee before, after the halving it would rise to around $20. So then the problem arises as you mentioned for micro transactions we can't really use Bitcoin. Thus wouldn't Bitcoin lose its utility as a currency?
Unless we start thinking of currency in terms of Bitcoin instead of converting it to dollars in our heads. In this case tx fees don't really change when block reward stops, they stay similar in terms of amount in BTC.