shawabawa3 already explained it "There would be an equilibrium where the fee is worth the marginal cost of mining it"

The miner has cost X and wants profit Y. He adds those together and now the price is X+Y. Someone wants his transaction on the blockchain, he is going to pay X+Y unless there is a miner that offers an even lower rate.

You're making the assumption that the block reward is high enough to make transaction fees irrelevant, which is a trivial observation and not what we are interested in talking about here.