When all the bitcoins are mined, miners will be paid in transaction fees. As the number of bit coins rewarded per block found decrease, the fees increase.
Correct. Now miners get 12.5 BTC + transaction fees per block. If we will propose, that there are 100 transactions in one block, extra 0,125 BTC is needed from every transaction to cover lack of block generation reward. Will miners bi interested to generate blocks for the transaction fees only without getting a reward?