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.The threat of a 51% attack has to do with the ability to double spend coins you already have and not the coins being paid as a reward for the mining being done.
If it was possible to succeed and to maintain a 51% attack, you could spend the coins you own at one place and spend the same coins at another place. This is not a option, because the cost of maintaining a 51% attack, out weigh the benefits from doing it.