i am thinking what will happen when block halving reward will become negligible may be after 2050 or before that. even now itself miners had quit mining because profit dropped from 25 to 12.5 then think of 2050's situation.
That is the reality that it will closely to happen and if a transaction doesn't require fees then the miners will just stop mining. For it is not going to be profitable to them to make mining as sourrce as it is not going to be helpful for us if there is going to be an implementation that no fees are going to be ever made with our transactions.
Halving will increase the price, so in the end they will earn the same amount when they convert to fiat also they are still holding "old" coins.
Fees will depend of the adoptance of the bitcoin, in 2050 it can be dead or be with high fees.