At the moment the miners for a block are awarded 12.5 BTC. Every 4 years this value will fall by half. For example, let's imagine that the prize value is 1.5 BTC for each block and that BTC's value against the dollar does not increase. In such a case, mining will not be very profitable.
At the same time, miners are also benefiting from the value of the fee. This time the fee values will increase. Then people will have to pay a high fee for bitcoin transfer. The biggest advantage of the Bitcoin system against banking systems was that they could shop without paying commissions to any brokerage house. If this happens, can bitcoin preference be reduced?
Basically bit coin is made up with proper programming which have done with considering future requirement. Already many things were predicted about Bitcoin environment which are really sustainable as well as complementary. Bitcoin is really invention made up of this century.
And as already predicted price of Bitcoin in future can give good return on investment with mining of Bitcoin. Even halving if decrease price of block reward, Still price matter. Because increasing price of Bitcoin giving returns to miners. And in future if reward decreases as per prediction still miner will be profitable if they have high end mining hardwares.