Transaction fees Will be the mining reward. High use of the bitcoin network Will be required to adequately pay the miners. On the other hand, You Will probably not live by that Time anymore, so You dont need to worry too much.
So the mining power thrown at bitcoin will be radically reduced at that point but it won't matter as 51% attacks would be useless since no new coins will be produced.
never radically because coin mining deacrease slowly...and the market will regulate all the existing coins...