You mean in 120 years from now? o_O

The answer is that bitcoin should've taken over world commerce then and miners fees will have long since eclipsed mining fees. At some point there's an inversion that takes place, where miners fees become bigger than the block reward on average.

I wonder when we'll see that happen for the first time.

Mining fees right now occasionally add up to one entire bitcoin. Perhaps in the year to come we'll see that figure double or triple as commerce takes off.